transport(tcp): remove sendMany function

[csegarragonz]

Turns out that sendMany becomes very convoluted when we need to handle partial writes. As it is not very clear the benefit that we are getting from it, and we may get away without it, we remove it in this PR.

I also fix the handling of partial writes for the sendOne method.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 82.27%. Comparing base (0c3120d) to head (1464836).

Files	Patch %	Lines
src/mpi/MpiWorld.cpp	42.85%	8 Missing ⚠️
src/transport/tcp/RecvSocket.cpp	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #423      +/-   ##
==========================================
+ Coverage   82.23%   82.27%   +0.04%     
==========================================
  Files         116      115       -1     
  Lines        7570     7556      -14     
==========================================
- Hits         6225     6217       -8     
+ Misses       1345     1339       -6     

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