Initial p2p channel implementation

[anjmao]

This PR adds initial p2p implementation in it's own standalone package. It uses H2C (HTTP2 over plain text) with kcp-go UDP transport and AES encryption. For now API accepts preshared private key and secure key exchange will be implemented in separate PR. It's possible (but not restricted) to send data in Protobuf format.

See channel_example_test.go for examples on proposed API.

[codecov-io]

Codecov Report

Merging #1862 into master will increase coverage by 0.34%.
The diff coverage is 78.62%.

@@            Coverage Diff             @@
##           master    #1862      +/-   ##
==========================================
+ Coverage   46.79%   47.13%   +0.34%     
==========================================
  Files         284      288       +4     
  Lines       11649    11780     +131     
==========================================
+ Hits         5451     5553     +102     
- Misses       5775     5795      +20     
- Partials      423      432       +9

Impacted Files	Coverage Δ
p2p/key.go	100% <100%> (ø)
p2p/context.go	100% <100%> (ø)
p2p/message.go	62.5% <62.5%> (ø)
p2p/channel.go	76.63% <76.63%> (ø)
communication/nats/dialog/dialog_waiter.go	86.11% <0%> (-2.78%)	⬇️
core/discovery/discovery.go	65.54% <0%> (-0.85%)	⬇️
tequilapi/sse/sse.go	71.11% <0%> (ø)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 920536f...dddc83c. Read the comment docs.