refactor(rust): Fix race condition in DistributorChannel

[orlp]

The old implementation was racy and could miss the last message(s) sent by the sender right before closing. It would check if there are messages available, and if not if the channel is closed, but between these two checks new messages could've come in.

This only affected the new streaming engine.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 27 lines in your changes missing coverage. Please review.

Project coverage is 79.87%. Comparing base (ab5200d) to head (6da7881).
Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
...stream/src/async_primitives/distributor_channel.rs	0.00%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19033      +/-   ##
==========================================
+ Coverage   79.85%   79.87%   +0.01%     
==========================================
  Files        1524     1524              
  Lines      207734   207749      +15     
  Branches     2908     2908              
==========================================
+ Hits       165891   165938      +47     
+ Misses      41294    41262      -32     
  Partials      549      549              

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