feat: adds support for blpop and brpop commands
fix: bpop tests expect correct returns
Merge branch 'bpop' of git://github.com/tim-dev/mockredis into tim-dev-bpop
Added mulit-key support in blpop and brpop commands. Added blocking_timeout attribute. Added brpoplpush command.
Using elasped time instead of sleep to check the timeout.
Minor refactor.

SS-3521
Added small delay for each iteration to avoid high cpu utilization.
SS-3521