* sliding-sync: spider all rooms on the user's account for search
On startup, slowly accumulate room metadata for all rooms on the
user's account. This is so we can populate the local search cache
with enough data for it to function, obviating the need to have
separate code paths for sliding sync searches.
This will allow spotlight search to work with slow/no network
connectivity, though clicking on the room will still require a
round trip.
This is an explicit request from @ara4n to improve the snapiness
of room searches, despite the unbounded bandwidth costs requesting
all N rooms on the user's account.
* Comments and tweak defaults
* Review comments; remove SS search code
* bugfix: use setListRanges once the list has been set up
If we don't, then we send needless extra data and can cause
bugs because setList will wipe the index->room_id map, which
trips up SlidingRoomListStore.
