[bitswap/peermanager] take read-lock for read-only operation

[gammazero]

Previously a read-only PeerManager operation was was protected by write-lock. This could have caused for the operation to block unnecessarily waiting for a write-lock.

There was also an instance of this with the PeerManager's session lock.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.43%. Comparing base (2a26bf9) to head (274f634).

Files with missing lines	Patch %	Lines
...tswap/client/internal/session/sessionwantsender.go	0.00%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main     #755      +/-   ##
==========================================
+ Coverage   60.32%   60.43%   +0.10%     
==========================================
  Files         245      245              
  Lines       31115    31112       -3     
==========================================
+ Hits        18771    18802      +31     
+ Misses      10670    10642      -28     
+ Partials     1674     1668       -6     

Files with missing lines	Coverage Δ
bitswap/client/internal/peermanager/peermanager.go	91.85% <100.00%> (ø)
...tswap/client/internal/session/sessionwantsender.go	96.94% <0.00%> (+0.91%)	⬆️

... and 12 files with indirect coverage changes