[core] Fixed order of group member state change before write ready

[ethouris]

The problem:

Write-readiness was set before the group state was upgraded from PENDING to IDLE. This brought a risk that the connecting function could exit before this state is changed and also the sending function might start the operation, catching all links PENDING, which resulted in error, even though the links are connected and the sending operation should at least try to send.

[codecov]

Codecov Report

Merging #1695 (1f199a3) into master (fda8b7b) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1695      +/-   ##
==========================================
- Coverage   60.51%   60.45%   -0.07%     
==========================================
  Files          78       78              
  Lines       17444    17444              
==========================================
- Hits        10556    10545      -11     
- Misses       6888     6899      +11     

Impacted Files	Coverage Δ
srtcore/core.cpp	54.86% <100.00%> (-0.05%)	⬇️
srtcore/congctl.cpp	81.02% <0.00%> (-3.08%)	⬇️
srtcore/window.cpp	81.81% <0.00%> (-1.30%)	⬇️
srtcore/list.cpp	76.23% <0.00%> (-0.58%)	⬇️
srtcore/api.cpp	48.66% <0.00%> (-0.06%)	⬇️
test/test_bonding.cpp	98.84% <0.00%> (+0.57%)	⬆️

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