multi: Add dynamic profile server infrastructure.

[davecgh]

This adds support for dynamically starting and stopping the profile server via the RPC server by introducing two new rpc commands named startprofiler and stopprofiler and updates the JSON-RPC API documentation accordingly.

It also updates the code that allows the profile to be started via the --profile command line / profile= config file option to use the new infrastructure such that it retains the existing behavior.

As a part of the changes, it expands the capabilities of the aforementioned profile options to allow specifying an interface name so it is more consistent with the other options that deal with listeners.

---

A high level overview of the changes are:

• Refactors profile listening port normalization and validation
  • Helps ensure consistent behavior regardless of how the profile server listen address is obtained
• Introduces a profile server type with methods to dynamically start, stop, and query the profile server
  • Includes a new flag to control whether or not non loopback addresses are allowed for future dynamic control
• Adds support for resolving interface names for the profile server listen address to make the behavior more consistent
• Adds startprofiler and stopprofiler JSON-RPC types
  • Adds serialization tests for the new types
• Implements the startprofiler and stopprofiler RPC methods and adds required help strings
  • Adds tests for the new handlers to help ensure they behave as intended
• Updates the JSON-RPC API documentation to add the new startprofiler and stopprofiler RPC methods

---

This consists of a series of commits to help ease the review process. Each commit is intended to be a self-contained and logically easy to follow change such that the code continues to compile and the tests continue to pass at each step.

See the description of each commit for further details.

---

Example commands showing the changes in action:

$ dcrctl startprofiler 127.0.0.1:5959
{
  "listeners": [
    "127.0.0.1:5959"
  ]
}

# Unable to change port without stopping running profile server first.
$ dcrctl startprofiler 127.0.0.1:1234
-26: profile server is already running

$ go tool pprof http://127.0.0.1:5959
Fetching profile over HTTP from http://127.0.0.1:5959/debug/pprof/profile
...
(pprof) top
Showing nodes accounting for 90ms, 100% of 90ms total
Showing top 10 nodes out of 38
      flat  flat%   sum%        cum   cum%
      40ms 44.44% 44.44%       40ms 44.44%  runtime.stdcall1
      10ms 11.11% 55.56%       10ms 11.11%  github.com/syndtr/goleveldb/leveldb.(*DB).newSnapshot
...

$ dcrctl stopprofiler
profile server stopped

$ dcrctl stopprofiler
-26: profile server is not started

# Unable to listen on non loopback addresses without flag.
$ dcrctl startprofiler :5959
-8: unable to start profile server: not permitted to listen on non loopback address ":5959" without setting the flag to allow it

$ dcrctl startprofiler :5959 true
{
  "listeners": [
    "0.0.0.0:5959",
    "[::]:5959"
  ]
}

$ dcrctl stopprofiler
profile server stopped

# Using a raw port as the address prepends "127.0.0.1:".
$ dcrctl startprofiler 5959
{
  "listeners": [
    "127.0.0.1:5959"
  ]
}