[core] Fixed guard for rcv-rexmit fields

[ethouris]

Problem: The following fields should be declared as guarded by m_RecvAckLock:

• m_iFlowWindowSize
• m_iSndLastAck
• m_tsLastRspTime
• m_iSndLastAck

They are already guarded in other places; this fix makes the guard consistent, including in places where these data are being read.

Tested with a trial transmission with the use of broadcast group. No deadlocks found and the lock ordering rule described in LowLevelInfo is preserved.

[codecov-commenter]

Codecov Report

Merging #1859 (bd86d52) into master (bc5a642) will increase coverage by 0.03%.
The diff coverage is 67.42%.

@@            Coverage Diff             @@
##           master    #1859      +/-   ##
==========================================
+ Coverage   64.57%   64.61%   +0.03%     
==========================================
  Files          91       91              
  Lines       19307    19317      +10     
==========================================
+ Hits        12467    12481      +14     
+ Misses       6840     6836       -4     

Impacted Files	Coverage Δ
srtcore/group.cpp	36.96% <12.50%> (ø)
srtcore/core.h	79.26% <50.00%> (ø)
srtcore/core.cpp	59.59% <63.33%> (+0.14%)	⬆️
srtcore/api.cpp	51.47% <68.14%> (-0.03%)	⬇️
test/test_file_transmission.cpp	95.65% <93.33%> (-0.69%)	⬇️
srtcore/api.h	96.87% <100.00%> (+0.44%)	⬆️
srtcore/sync.cpp	86.66% <0.00%> (-2.23%)	⬇️
srtcore/congctl.cpp	78.64% <0.00%> (-2.09%)	⬇️
srtcore/queue.cpp	79.68% <0.00%> (-1.46%)	⬇️
test/test_bonding.cpp	97.84% <0.00%> (-0.54%)	⬇️
... and 6 more

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