Synapse retries too-large appservice transactions indefinitely

[Half-Shot]

The flow goes more or less like this:

1. The bridge gets behind on transaction traffic.
2. A large number of events build up on the synapse side, and get bundled into one large transaction.
3. Synapse tries to send this transaction, and the bridge responds with a 413 (payload too large). matrix-org bridges are limited to 5MB transactions.
4. This leads to appservice_stream_position can get into such a state that ASes break silently #1834.

(FTR, the txn 355792220 for the irc-freenode bridge contained 548 events)

We probably want to limit the number of events send in a transaction on the synapse side, and then ensure the bridge can handle (LIMIT*65k + padding).

[richvdh]

#3536