bitswap: messagequeue: lock only needed sections

[hsanjuan]

A couple of locks here seem to lock also while doing things like sending things on channels. This seems prone to deadlocks and is wasteful.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.48%. Comparing base (6397847) to head (a3e4570).
Report is 1 commits behind head on main.

@@            Coverage Diff             @@
##             main     #787      +/-   ##
==========================================
+ Coverage   60.46%   60.48%   +0.01%     
==========================================
  Files         245      245              
  Lines       31133    31138       +5     
==========================================
+ Hits        18826    18834       +8     
- Misses      10627    10629       +2     
+ Partials     1680     1675       -5     

Files with missing lines	Coverage Δ
...tswap/client/internal/messagequeue/messagequeue.go	85.44% <100.00%> (+0.13%)	⬆️

... and 13 files with indirect coverage changes

[gammazero]

The last unlock may be important if this ever blocks:

mq.events <- messageFinishedSending