Enable multisharding for solid queue

[alxckn]

Fixes #353

This is a proposal to add multisharding as a supported configuration option for Solid Queue. It makes it possible to easily migrate incrementally an application from one to multiple shards as changing the connects_to configuration will not change any of the default behavior of Solid Queue.

This introduces 2 ways to determine which shard to enqueue a job to:

• specify a configured queue_adapter on a job, so that a given job class would be assigned to a given shard (ensuring full compatibility with some other Solid Queue features such as concurrency control)
• specify a global shard_selection_lambda to have complete control over shard selection logic when enqueueing jobs

By configuring the active_shard dynamically (for instance using an environment variable), it is possible to start workers, dispatchers and schedulers that will consume each shard.