This bug has been found by @bnjbvr, all the credits go to him. I've just
added some comments around his code.
Prior to this patch, the room unsubscription buffer
(`SlidingSync::room_unsubscriptions`) was reset before the request was
sent. So if something went wrong, the next request would not include the
room unsubscriptions.
This patch updates this behavior. First, it replaces `Vec` by `HashSet`
to avoid a O(n^2) look up.
Second, a copy of room unsubscriptions used by the request is kept, so
that it can be used to cherry-pick which room unsubscription to remove
from the buffer once a response from the server is received. It's
important to not clear the entire room unsubscriptions buffer as more
unsubscriptions could have been inserted meanwhile.
Ivan Enderlin
d8a9408efe