Use local seshat index for backscroll/backfill instead of hitting the server

### Your use case

#### What would you like to do?
Instead of hitting the server for events, why not use the local seshat index when backscrolling in an E2EE room if you have seshat enabled?

#### Why would you like to do it?
- Saves bandwidth
- Lessens load on server
- Much faster experience when backscrolling

#### How would you like to achieve it?
Take some of the search logic and use it to get events instead of issuing requests to the server if E2EE is enabled and the user has seshat.

### Have you considered any alternatives?

None

### Additional context

Nil