Fix potential race condition

[ryanolson]

Prior to this PR, there was a possible race condition that could be hit by two execution paths where,

• path 1 calls Server::drop_stream which is a yielding call
• path 2 call Server:m_connections->drop_all_streams();

Server::drop_stream locks the server's state, but Server::m_connections is not protected by that mutex.

Relates to #247

[ryanolson]

Note, this will pass CI, because CI is still skipping test_mrc_private.x.

[ryanolson]

Note: a boost::fibers::recursive_mutex is not an option due to the condition variable

[codecov]

Codecov Report

Merging #248 (39dc6fe) into branch-23.01 (6f7b4ad) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-23.01     #248      +/-   ##
================================================
- Coverage         46.00%   45.96%   -0.04%     
================================================
  Files               335      335              
  Lines              9278     9288      +10     
  Branches            774      775       +1     
================================================
+ Hits               4268     4269       +1     
- Misses             5010     5019       +9     

Flag	Coverage Δ
cpp	37.58% <0.00%> (-0.05%)	⬇️
py	39.21% <0.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cpp/mrc/src/internal/control_plane/server.cpp	0.00% <0.00%> (ø)
cpp/mrc/src/internal/control_plane/server.hpp	0.00% <ø> (ø)
...ternal/control_plane/server/connection_manager.cpp	0.00% <0.00%> (ø)
cpp/mrc/src/internal/resources/manager.cpp	54.02% <ø> (ø)
cpp/mrc/src/internal/system/fiber_task_queue.cpp	65.00% <0.00%> (+2.50%)	⬆️

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[ryanolson]

@gpucibot merge