Fix Agent close hanging

[JoTurk]

Description

Abort started candidate i/o before waiting for the task loop to stop.
this prevents close from deadblocking when a task is blocked in WriteTo
This is a fix i have been testing for a few days it's missing regression test and some improvements.

[JoTurk]

@boks1971 I'll get this improved ready for review + regressions tests by tomorrow.

[codecov]

Codecov Report

❌ Patch coverage is 85.24590% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.42%. Comparing base (772c9a1) to head (bfe94dc).

Files with missing lines	Patch %	Lines
candidate_base.go	68.42%	3 Missing and 3 partials ⚠️
udp_mux.go	62.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #934      +/-   ##
==========================================
- Coverage   88.51%   88.42%   -0.09%     
==========================================
  Files          46       46              
  Lines        5973     6014      +41     
==========================================
+ Hits         5287     5318      +31     
- Misses        473      480       +7     
- Partials      213      216       +3     

Flag	Coverage Δ
go	88.42% <85.24%> (-0.09%)	⬇️

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