Add timeout support on send

[egorse]

Add internal/errorgrp package to support cancellable error groups Add tests for push/pull timeout

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e16dc3e) 68.51% compared to head (2bf3109) 68.07%.

Files	Patch %	Lines
rep.go	50.00%	1 Missing :warning:
req.go	50.00%	1 Missing :warning:
socket.go	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #148 +/- ## ========================================== - Coverage 68.51% 68.07% -0.44% ========================================== Files 29 30 +1 Lines 2560 2600 +40 ========================================== + Hits 1754 1770 +16 - Misses 704 727 +23 - Partials 102 103 +1 ```

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[egorse]

we don't exercize the whole errorgrp.Group2.Go code. we could probably try to make sure we still get the context error when it expires during the execution of f. (probably with a 2-level handshake where the cancel() happens when we are sure f is being executed)

This is sort of odd - I had impression that only block execution of f if context is already cancelled (i.e. if the eg has limits on number of executable functions and parent context got cancelled prior next f in eg started). I will check it and update PR (include others suggestions)

update: Test case was missing

[egorse]

I think its ready for next review cycle. Please comment

[egorse]

Please take a look. Still not sure how well it is on implicit errgroup.Group initialization

[egorse]

Dough - original init() was nicer for codecov. Will wait for your comments