Ticket repurposed following CR discussion.
PersistentTaskStore dependencies have always been indirectly compiled by other targets, resulting sometimes in an unclear dependency graph: for example, those two dependencies are necessary to use PersistentTaskStore, but it's not straightforward to understand why VectorClockMongoD depends on them.

Since there are more targets "secretly" depending on PersistentTaskStore, it would be useful to better modularize it by creating a persistent_task_store SCons target and refer it in dependent libraries.

As the component is not just sharding related:

• Move the files from db/s to db.
• Make PersistentTaskStoreTest not extend ShardServerTestFixture and not refer any sharding-specific feature.