limit concurrency when closing all connections handled by a Transport

[marten-seemann]

I'm not 100% sure we not a Go routine here in the first place, but limiting the concurrency definitely seems reasonable.

[codecov]

Codecov Report

Merging #4103 (50b97b0) into master (9a397ab) will decrease coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4103      +/-   ##
==========================================
- Coverage   83.74%   83.72%   -0.02%     
==========================================
  Files         150      150              
  Lines       15394    15397       +3     
==========================================
  Hits        12891    12891              
- Misses       2003     2006       +3     
  Partials      500      500              

Files Changed	Coverage Δ
packet_handler_map.go	84.80% <100.00%> (+0.27%)	⬆️

... and 1 file with indirect coverage changes

[sukunrt]

Change seems okay but I'm not sure what is the deadlock leading to the test failures. The only difference I can think of is previously we released packatHandlerMap mutex and let the goroutines run to completion and now we don't release the mutex.

[marten-seemann]

The problem is that the connections remove their connection IDs from the map, which requires them to take the mutex which we're now still holding.