Preparations to actually remove/replace `BaseDendrite`.
Quite a few changes:
- SyncAPI accepts an `fulltext.Indexer` interface (fulltext is removed
from `BaseDendrite`)
- Caches are removed from `BaseDendrite`
- Introduces a `Router` struct (likely to change)
- also fixes#2903
- Introduces a `sqlutil.ConnectionManager`, which should remove
`base.DatabaseConnection` later on
- probably more
* Merge user API databases into one
* Remove DeviceDatabase from config
* Fix tests
* Try that again
* Clean up keyserver device keys when the devices no longer exist in the user API
* Tweak ordering
* Fix UserExists flag, device check
* Allow including empty entries so we can clean them up
* Remove logging
