feat: server reliability improvements

[kincaidoneil]

Improvements:

• Safely closing the server. Waits for all incoming ILP Prepare packets currently being handled to be returned + 100ms before disconnecting the plugin and resolving Server.close(). Also added a test that attempts to create this race condition.
• Callback/custom logic before fulfilling packets. Enables a shouldFulfill callback function to be passed to the connection or server, which will be called last before fulfilling an incoming ILP Prepare with money. This enables applications to perform async operations to account for the incoming packet in their own system, and/or choose to fulfill or reject any individual packet. A couple minor questions:
  • I changed totalReceived so it no longer throws if there's an overflow, since that's the only case where the packet wouldn't actually be fulfilled if shouldFulfill resolved successfully. Is this behavior okay?
  • To uniquely track each packet, the connectionId (last segment of ILP address, including the connection tag) and sequence number are passed to the callback. Should the connection tag be passed to the callback instead?

Also: fixes vulnerabilities, and updates the logging so the same connection ID is used between the client & server, by using the token in the destination ILP address.

[codecov-io]

Codecov Report

Merging #136 into master will decrease coverage by 2.17%. The diff coverage is 97.05%.

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
- Coverage   91.26%   89.09%   -2.18%     
==========================================
  Files          14       14              
  Lines        1500     1733     +233     
  Branches      108      268     +160     
==========================================
+ Hits         1369     1544     +175     
+ Misses        112      111       -1     
- Partials       19       78      +59     

Impacted Files	Coverage Δ
src/index.ts	83.33% <ø> (-1.97%)	:arrow_down:
src/pool.ts	100.00% <ø> (ø)
src/connection.ts	88.25% <94.73%> (-1.75%)	:arrow_down:
src/crypto.ts	100.00% <100.00%> (ø)
src/server.ts	85.71% <100.00%> (-1.79%)	:arrow_down:
src/util/rational.ts	75.00% <0.00%> (-6.64%)	:arrow_down:
src/stream.ts	87.45% <0.00%> (-4.41%)	:arrow_down:
src/util/congestion.ts	80.95% <0.00%> (-4.05%)	:arrow_down:
src/util/data-offset-sorter.ts	91.66% <0.00%> (-1.89%)	:arrow_down:
... and 7 more

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[kincaidoneil]

A couple changes:

1. A hold is applied to totalReceived before calling shouldFulfill, which resolves the overflow issue and prevents a race condition.
2. I changed shouldFulfill so it's provided a unique packet ID, which is generated via hmac(sharedSecret, "ilp_stream_packet_id" + sequence). This allows shouldFulfill to be used even if there's no connection tag, is a little more intuitive for developers, and maybe resolves #131.

Ready for re-review.

[kincaidoneil]

Right now the packetId is serialized as UTF-8. Should it be serialized differently (base64? hex?) or should the callback just be given the buffer directly so it the developer can choose how they want to serialize it?

[sharafian]

Right now the packetId is serialized as UTF-8

This seems fine to be so long as it has a reasonable text representation. If there are values that don't represent actual character values then let's use a buffer rather than base64 or hex