Add db_conn parameters to the `__init__` methods of the *Store classes, so that
they are all consistent, which makes the multiple inheritance work correctly
(and so that we can later extract mixins which can be used in the slavedstores)
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.
Using _simple_select_list is fairly expensive for functions that return
a lot of rows and/or get called a lot. (This is because it carefully
constructs a list of dicts).
get_current_state_ids gets called a lot on startup and e.g. when the IRC
bridge decided to send tonnes of joins/leaves (as it invalidates the
cache). We therefore replace it with a custon txn function that builds
up the final result dict without building up and intermediate
representation.
* `get_forward_extremeties_for_room` takes a numeric `stream_ordering`. We were
passing a `RoomStreamToken`, which meant that it returned the *current*
extremities, rather than those corresponding to the `from_token`. However:
* `get_state_ids_for_events` required a second ('types') parameter; this meant
that a `TypeError` was thrown and we ended up acting as though there was *no*
prev state.
* `get_state_ids_for_events` actually returns a map from event_id to state
dictionary - just looking up the state keys in it again meant that we acted
as though there was no prev state. We now check if each member's state has
changed since *any* of the extremities.
Also add/fix some comments.
