avoid cancelling send future

[arnetheduck]

otherwise, the use of asyncSpawn results in breakage

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 82.51%. Comparing base (49a92e5) to head (5ed400d).
Report is 2 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1075      +/-   ##
============================================
- Coverage     82.57%   82.51%   -0.07%     
============================================
  Files            91       91              
  Lines         15812    15848      +36     
============================================
+ Hits          13057    13077      +20     
- Misses         2755     2771      +16     

Files	Coverage Δ
libp2p/protocols/pubsub/pubsubpeer.nim	88.23% <0.00%> (-0.35%)	⬇️

... and 9 files with indirect coverage changes

[diegomrsantos]

supersedes #1074

[diegomrsantos]

@arnetheduck my local nimbus node crashed when running using unstable:

libbacktrace error: no debugging symbols available. Compile with '--debugger:native'.
Error: unhandled exception: Asynchronous task [sendMsgSlow() at pubsubpeer.nim:303] was cancelled! [FutureDefect]