Dual-channel-replication: set connection to blocking in child process

[xbasel]

Dual-channel file descriptors are set to blocking mode in the main thread, followed by a blocking write. This sets the file descriptors to non-blocking if TLS is used (see connTLSSyncWrite()).

The child process assumes all file descriptors are in blocking mode, so a short-write leads to errors in the replica while loading the RDB.

This change sets file descriptors to blocking in the child process.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.38%. Comparing base (b728e41) to head (f0aaae3). Report is 50 commits behind head on unstable.

Files	Patch %	Lines
src/replication.c	50.00%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #862      +/-   ##
============================================
- Coverage     70.47%   70.38%   -0.09%     
============================================
  Files           112      112              
  Lines         61467    61471       +4     
============================================
- Hits          43320    43269      -51     
- Misses        18147    18202      +55     

Files	Coverage Δ
src/rdb.c	76.24% <100.00%> (-0.12%)	:arrow_down:
src/replication.c	87.18% <50.00%> (-0.17%)	:arrow_down:

... and 11 files with indirect coverage changes

[madolson]

@xbasel We can close this right?

[enjoy-binbin]

i see #837 does not have this change, do we still need it? @naglera

[ranshid]

closing as was already implemented as part of #837