Add ability to configure global Redis namespace prefix

[dammer]

Currently, there is no way to configure a global Redis namespace prefix in Mosquito.
This poses challenges when running multiple instances of the application in cloud environments like AWS with shared Redis instance.
Since I want each application instance to have its own execution context, we are currently facing an issue of Redis worker overlap between containers, which can lead to tasks being unexpectedly executed from other containers.

Please include some details:

Crystal version: 1.11.2
Mosquito Shard version: 1.0.2

[dammer]

Many thanks to @robacarp for the quick response and helpful remarks. It's great to improve such an excellent product!