I hope the comments explain the changes.
`notifyWorkers` notifies a worker which then calls `processServer`,
which in turn gets all users and calls `processServerUser`. There is no
need to call `processServer` for the same domain on startup.
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
