Clients rely on transaction IDs coming down their /sync streams so they
can pair up an incoming event with an event they just sent but have not
yet got the event ID for.
The proxy has not historically handled this because of the shared work
model of operation, where we store exactly 1 copy of the event in the
database and no more. This means if Alice and Bob are running in the
same proxy, then Alice sends a message, Bob's /sync stream may get the
event first and that will NOT contain the `transaction_id`. This then
gets written into the database. Later when Alice /syncs, she will not
get the `transaction_id` for her event which she sent.
This commit fixes this by having a TTL cache which maps (user, event)
-> txn_id. Transaction IDs are inherently ephemeral, so keeping the
last 5 minutes worth of txn IDs in-memory is an easy solution which
will be good enough for the proxy. Actual server implementations of
sliding sync will be able to trivially deal with this behaviour natively.
