Prevent "send on closed channel" panic in purger goroutine

[alitto]

Changes included

• Prevent panic thrown in purger goroutine when attempting to stop a worker after tasks channel is closed (reported in https://github.com/alitto/pond/pull/26).
• Ensure calls to StopAndWait() from multiple goroutines concurrently makes them all wait until all tasks have completed.

[codecov-commenter]

Codecov Report

Merging #27 (fe39f93) into master (db6c2ff) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #27   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files           2        2           
  Lines         325      328    +3     
=======================================
+ Hits          324      327    +3     
  Misses          1        1           

Impacted Files	Coverage Δ
pond.go	99.67% <100.00%> (+<0.01%)	:arrow_up:

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[chenyahui]

1. maybe stop() function forget to close(p.tasks)?
2. It is possible to wait the purger goroutine exit in stop function?

I implement it in this commit https://github.com/alitto/pond/pull/27/commits/fe39f934963a60e73a75960c64b931584fed70d8 at the same git branch.

Mind to see if this is the right way?

[alitto]

This is great @chenyahui, thank you so much for your contributions :slightly_smiling_face: I'll merge this and release a new patch version (1.7.1)