Add callback API to track allocation lifeycle

[rg0now]

Fixes https://github.com/pion/turn/issues/324, https://github.com/pion/turn/pull/325, and https://github.com/pion/turn/pull/402

This PR adds a set of callbacks that can be used to track the lifecycle of TURN allocations in the server. This allows user code to be called whenever an authentication request has been processed (this can be used to mitigate DoS attacks against a server, see https://github.com/pion/turn/pull/402), when an allocation has been created/removed (e.g., to implement user tracking for adding quota support, see https://github.com/pion/turn/issues/324), when a permission has been created/removed (e.g., to log the peers a user tries to reach via the server), a channel has been created/removed (this will simplify integrating external TURN accelerators, see https://github.com/pion/turn/pull/360), or when an allocation ends with an error.

The implementation adds a new EventHandlers struct to the ServerConfig, defined as follows:

type EventHandlers struct {
  OnAuth func(srcAddr, dstAddr net.Addr, protocol, username, realm string, method string, verdict bool)
  OnAllocationCreated func(srcAddr, dstAddr net.Addr, protocol, username, realm string, requestedPort int)
  OnAllocationDeleted func(srcAddr, dstAddr net.Addr, protocol, username, realm string)
  OnAllocationError func(srcAddr, dstAddr net.Addr, protocol, message string)
  OnPermissionCreated func(srcAddr, dstAddr net.Addr, protocol, username, realm string, peer net.IP)
  OnPermissionDeleted func(srcAddr, dstAddr net.Addr, protocol, username, realm string, peer net.IP)
  OnChannelCreated func(srcAddr, dstAddr net.Addr, protocol, username, realm string, peer net.Addr, channelNumber uint16)
  OnChannelDeleted func(srcAddr, dstAddr net.Addr, protocol, username, realm string, peer net.Addr, channelNumber uint16)
}

It is OK to handle only a subset of the events.

Caveats:

• Event handlers are called synchronously, so long-running callbacks will block the server.
• Theoretically, allocations are allowed to change credentials during their lifetime, in that any Refresh, CreatePermission or ChannelBind message may use a different credential. Since this does not seem to be easy to implement with the PeerConnection API, credential swaps are currently no tracked.

[rg0now]

CC @renandincer

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.86294% with 18 lines in your changes missing coverage. Please review.

Project coverage is 68.62%. Comparing base (cc03474) to head (dae4d0c). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/server/util.go	73.07%	5 Missing and 2 partials :warning:
internal/allocation/five_tuple.go	60.00%	4 Missing :warning:
server.go	33.33%	3 Missing and 1 partial :warning:
internal/server/nonce.go	50.00%	0 Missing and 1 partial :warning:
relay_address_generator_range.go	0.00%	1 Missing :warning:
server_config.go	97.43%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #419 +/- ## ========================================== + Coverage 68.15% 68.62% +0.47% ========================================== Files 43 43 Lines 2352 3095 +743 ========================================== + Hits 1603 2124 +521 - Misses 582 804 +222 Partials 167 167 ``` | [Flag](https://app.codecov.io/gh/pion/turn/pull/419/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [go](https://app.codecov.io/gh/pion/turn/pull/419/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `68.62% <90.86%> (+0.47%)` | :arrow_up: | | [wasm](https://app.codecov.io/gh/pion/turn/pull/419/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `25.62% <3.04%> (-2.27%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#carryforward-flags-in-the-pull-request-comment) to find out more.

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[renandincer]

I just looked at it briefly @rg0now! Thank you for PRing this in.

I think it might be useful to get a handle to the PacketConn from OnAllocatedCreated as well. This might be useful for rate limiting. What do you think?

[rg0now]

I think a better way to handle rate limiting would be to provide a RelayAddressGenerator.AllocatePacketConn to the server that will emit rate-limited packetconns. This works even today. The only issue with that approach is if we need user context for applying rate limiting (i.e., when different users may get different rate limits), since currently we miss the user context in the RelayAddressGenerator interface handlers. But then again, it would somehow be more adequate to add the user context there. Wdyt?

[renandincer]

Agreed. Only issue is that would be a breaking API change.