Handle expired timers in basic_stream transfer_op

[ashtum]

Previously, the code only handled expired timers when the buffer was empty.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.02%. Comparing base (95f0cfb) to head (ff3244a).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2866      +/-   ##
===========================================
- Coverage    93.05%   93.02%   -0.04%     
===========================================
  Files          178      178              
  Lines        13688    13694       +6     
===========================================
+ Hits         12738    12739       +1     
- Misses         950      955       +5     

Files	Coverage Δ
include/boost/beast/core/detail/stream_base.hpp	100.00% <100.00%> (ø)
include/boost/beast/core/impl/basic_stream.hpp	85.61% <100.00%> (-0.50%)	⬇️

... and 2 files with indirect coverage changes

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