[Redis Broker] Wait for currently processing tasks to finish before closing the redis pool

[gow]

The redis broker sometimes shuts down before currently processing tasks are complete. This typically is not an issue except when these running tasks wish to publish other tasks before completing. Since the StopConsuming() method closes the Redis pool, publishing of new tasks fail as well.

This PR waits for the tasks currently being processed before closing the redis pool.

[codecov]

Codecov Report

Merging #552 into master will increase coverage by 1.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   38.15%   39.19%   +1.03%     
==========================================
  Files          32       32              
  Lines        3310     3345      +35     
==========================================
+ Hits         1263     1311      +48     
+ Misses       1908     1892      -16     
- Partials      139      142       +3     

Impacted Files	Coverage Δ
v1/backends/redis/redis.go	69.39% <0.00%> (-0.55%)	⬇️
v1/worker.go	3.39% <0.00%> (-0.18%)	⬇️
v2/worker.go	3.39% <0.00%> (+1.30%)	⬆️
v2/server.go	26.89% <0.00%> (+26.89%)	⬆️

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