Make get_state_groups_from_groups faster.

Fix users claimed non-exclusively by an app service don't get notific…

Most of the time was spent copying a dict to filter out sentinel values
that indicated that keys did not exist in the dict. The sentinel values
were added to ensure that we cached the non-existence of keys.

By updating DictionaryCache to keep track of which keys were known to
not exist itself we can remove a dictionary copy.

Make get_joined_users faster when we have delta state

Speed up get_joined_hosts

Make presence use cached users/hosts in room

Speed up get_domain_from_id

Prefill state caches

Add an index to event_search

Configurable maximum number of events requested by /sync and /messages

Don't log exceptions for NotRetryingDestination

Documantation to chek synapse version 

* Added HS as property in SyncRestServlet
* Fixed set_timeline_upper_limit function implementat¡ion

Set the limit on the returned events in the timeline in the get and sync
operations. The default value is -1, means no upper limit.

For example, using `filter_timeline_limit: 5000`:

POST /_matrix/client/r0/user/user:id/filter
{
room: {
    timeline: {
      limit: 1000000000000000000
    }
}
}

GET /_matrix/client/r0/user/user:id/filter/filter:id

{
room: {
    timeline: {
      limit: 5000
    }
}
}

The server cuts down the room.timeline.limit.

... because the table is virtual

Log the number of events we will be deleting at info.

- to make the purge API quicker

When purging, don't de-delta state groups we're about to delete

Try to make this clearer with more comments and some variable renames