fix: track promise only after a successful sendRpc()

[twoeths]

Motivation

• In rare case, a sendRpc() call may be failed or got delayed, we should only track promise after a successful sendRpc

part of #413

[what-the-diff]

• Add a new variable sent to store the result of sendRpc()
• If iwantMessageIds is not undefined, add promise for this peer id and message ids in gossipTracer

[codecov-commenter]

Codecov Report

Patch coverage: 7.14% and project coverage change: -0.07 ⚠️

Comparison is base (1e1cf5a) 83.31% compared to head (e07d38a) 83.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
- Coverage   83.31%   83.24%   -0.07%     
==========================================
  Files          48       48              
  Lines       11800    11812      +12     
  Branches     1271     1272       +1     
==========================================
+ Hits         9831     9833       +2     
- Misses       1969     1979      +10     

Impacted Files	Coverage Δ
src/metrics.ts	21.13% <0.00%> (-0.12%)	⬇️
src/index.ts	70.34% <10.00%> (-0.13%)	⬇️

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[twoeths]

after 3.5h, there are more than 2k untracked promise due to unsuccessful sendRpc() call in a mainnet node with only Nimbus peers