[WIP] mRuby commands

[palkan]

What is the purpose of this pull request?

Add ability to perform Action Cable commands within anycable-go itself without calling RPC server.

That would allows us to increase RPS (requests-per-second) for RPC bridge (see the benchmarks).

To implement this caching we fetch (not implemented yet) the source code of the method and compile it within the embedded mRuby VM.

Benchmarks

Running anycable-go on the same machine as RPC server:

# websocket-bench echo
rpc_cached           95per-rtt:   5ms
rpc                  95per-rtt:   30ms

# controller.Perform
rpc_cached           0.05ms
rpc                  0.7ms

TODO

Features:

• [ ] Embed mRuby sources (https://github.com/gobuffalo/packr ?)
• [ ] Fetch method source from RPC
• [ ] Detect cacheable* commands
• [ ] Cache invalidation

* Not every channel command could be cached, of course. We need a way to either automatically guest whether it's possible to "move" the method to mRuby or to provide an API to mark such methods in the Ruby source code.

Misc:

• [x] mRuby memory profiling (GC arena save/restore?)
• [ ] Run benchmarks/stress test
• [ ] Add Changelog entry
• [ ] Update documentation for this change