fix(dcutr): update the DCUtR initiator transport direction to Inbound

[diegomrsantos]

During DCUtR, the listener initiates the direct connection (it is called the DCUtR client) attempt and must connect to the other peer as the incoming upgrade direction. Yamux must use this direction to decide on using odd or even stream IDs, not the initial underlying connection direction or socket "direction". It means we need to change the underlying TCP connection direction to Inbound, despite it being a dial.

This is an unusual case where a peer initiates a connection, but should treat it as incoming. The reason is probably that DCUtR happens when the other remote peer dials a peer's relayed address, therefore the original connection was in fact incoming.

Related:
https://github.com/libp2p/specs/tree/master/yamux
https://github.com/libp2p/specs/blob/master/relay/DCUtR.md

[codecov]

Codecov Report

Merging #994 (9d0cfd5) into unstable (ce0685c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 9d0cfd5 differs from pull request most recent head 9d0b90c. Consider uploading reports for the commit 9d0b90c to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##           unstable     #994   +/-   ##
=========================================
  Coverage     83.23%   83.24%           
=========================================
  Files            91       91           
  Lines         15328    15325    -3     
=========================================
- Hits          12759    12758    -1     
+ Misses         2569     2567    -2     

Files	Coverage Δ
libp2p/dial.nim	50.87% <100.00%> (ø)
libp2p/dialer.nim	92.76% <100.00%> (ø)
libp2p/protocols/connectivity/dcutr/client.nim	72.09% <100.00%> (ø)
libp2p/protocols/connectivity/dcutr/server.nim	72.09% <100.00%> (ø)
libp2p/protocols/secure/secure.nim	77.55% <100.00%> (-0.23%)	⬇️
libp2p/switch.nim	91.32% <100.00%> (ø)
libp2p/transports/transport.nim	87.75% <100.00%> (+0.25%)	⬆️
libp2p/upgrademngrs/muxedupgrade.nim	88.70% <100.00%> (+1.01%)	⬆️
libp2p/upgrademngrs/upgrade.nim	100.00% <100.00%> (+3.33%)	⬆️