fix(yamux): doesn't work in a Relayv2 connection

[diegomrsantos]

Trying to use the circuit relay v2 with Yamux raises some issues:

• The direction of the RelayConnection wasn't rightly set creating an error. To go deeper, this is a description of what happened:
  • A connects to Relay (A direction is outbound and Relay direction is inbound)
  • A reserves a slot on Relay
  • B connects to Relay (B direction is outbound and Relay direction is inbound)
  • B connects to A through the Relay (A and B direction were both outbound, which is wrong, causing an error to be raised).
• As we didn't make the YamuxChannel inherit from BufferStream, we had to manage the isEof which was not implemented. The main reason as to why we didn't use BufferStream is because the way it works would have made the creation of the YamuxChannel harder to implement (for example BufferStream manage its backpressure with the number of message received, YamuxChannel uses the number of bytes received).
• Some issues with the tests

[codecov]

Codecov Report

Merging #979 (d282f52) into unstable (fb05f5a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head d282f52 differs from pull request most recent head 1b001c0. Consider uploading reports for the commit 1b001c0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##           unstable     #979   +/-   ##
=========================================
  Coverage     83.19%   83.20%           
=========================================
  Files            91       91           
  Lines         15317    15328   +11     
=========================================
+ Hits          12743    12753   +10     
- Misses         2574     2575    +1     

Files	Coverage Δ
libp2p/builders.nim	94.44% <100.00%> (+0.19%)	⬆️
libp2p/muxers/yamux/yamux.nim	89.72% <100.00%> (+1.38%)	⬆️
libp2p/protocols/connectivity/relay/rconn.nim	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

[diegomrsantos]

Looks great, thanks a lot @lchenut.

[kaiserd]

LGTM ❇️

[kaiserd]

r: Relay = nil

Should we not generally use an Option in such cases?

[diegomrsantos]

For a test, I believe it's not critical. What are your thoughts?