Implement Persistent Jobs Service

[ecton]

Work in progress.

Closes #78

The goal of this pull request is to implement BonsaiDb's first "plugin". It may not actually be a plugin at the end of this pull request, but the long term vision is to allow the Jobs service to be reused independently of the one that will be built into BonsaiDb's admin database.

This would enable building individual pools of queues and workers that have different strategies for assigning jobs and distinct monitoring frontends, or allow developers to just use the one built into BonsaiDb.