feat: add publish and getpubsub for streaming

[FogDong]

Fix #437

This PR adds publish and getPubSub so that users can use publish to stream messages and use getPubSub to receive.

[kamikazechaser]

Thanks for this. From what I can see this is a feature to track tasks via Redis pub/sub. An alternative to ResultWriter.Write.

However this is not really a fix for #437 and looks very unrelated to it (capturing Redis commands). Could you create a new issue and maybe get feedback on it? Nevertheless, this looks like a good feature to have.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.

Project coverage is 68.12%. Comparing base (6b98c0b) to head (40df09c).
Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
asynq.go	0.00%	6 Missing ⚠️
internal/rdb/rdb.go	0.00%	6 Missing ⚠️
inspector.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #736      +/-   ##
==========================================
- Coverage   68.37%   68.12%   -0.25%     
==========================================
  Files          27       27              
  Lines        3836     3850      +14     
==========================================
  Hits         2623     2623              
- Misses        929      943      +14     
  Partials      284      284              

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