Upgrade flow refactoring

[Menduist]

This PR simplifies a lot the upgrade flow

The main thing is that upgradeIncoming and upgradeOutgoing now return the Muxer, that the switch / dialer will then pass to the connManager to be stored.
Previously, this relied on the upgrade manager storing the connection itself at the end of the process

Other things:

• connManager now only store the muxer (instead of muxer & connection). Connection is available in muxer.connection
• MultiStreamselect is now usable without holding the instance (since it's stateless anyway)
• Identify is now handled by the PeerStore instead of the upgrademanager
• Removed the "upgrade" future from the Connection which is no longer necessary
• Unified outgoing & incoming upgrade flows
• Simplified MuxerProvider

Sorry, big PR, but one thing lead to another

[codecov]

Codecov Report

Merging #807 (c9cc2bc) into unstable (c1a3bd8) will decrease coverage by 0.12%.
The diff coverage is 96.47%.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #807      +/-   ##
============================================
- Coverage     83.88%   83.77%   -0.12%     
============================================
  Files            86       86              
  Lines         15096    14969     -127     
============================================
- Hits          12663    12540     -123     
+ Misses         2433     2429       -4     

Impacted Files	Coverage Δ
libp2p/protocols/secure/secure.nim	71.15% <ø> (ø)
libp2p/muxers/muxer.nim	65.00% <57.14%> (-22.81%)	⬇️
libp2p/multistream.nim	95.34% <94.28%> (+0.54%)	⬆️
libp2p/dialer.nim	93.08% <97.05%> (+1.45%)	⬆️
libp2p/connmanager.nim	90.31% <97.56%> (+0.03%)	⬆️
libp2p/builders.nim	95.07% <100.00%> (-0.68%)	⬇️
libp2p/muxers/yamux/yamux.nim	88.39% <100.00%> (-1.07%)	⬇️
libp2p/peerstore.nim	97.89% <100.00%> (-2.11%)	⬇️
libp2p/protocols/connectivity/autonat/client.nim	99.66% <100.00%> (ø)
libp2p/protocols/connectivity/autonat/service.nim	95.65% <100.00%> (ø)
... and 25 more

[Menduist]

This PR is now stable on nimbus, anything else to change?