Move persist_events out from main data store.

[erikjohnston]

This is in preparation for splitting out of state_groups_state from the
main store into it own one, as persisting events depends on calculating
state.

Broadly I've just moved functions from the main data store up into an EventPersistenceStore and changed calls to storage functions to self.main_store.* and self.state_store.*

[erikjohnston]

I'm currently fiddling to change the way we access the persistence store ftr

[erikjohnston]

So the plan is broadly that all the new storage classes get added to the Storage class (accessed via get_storage) and that getting the main data store directly via get_datastore() is sort of deprecated?

[richvdh]

generally lgtm, though it's not easy to see from the diff what's happening. A few requests for docstrings etc