Fixes https://github.com/element-hq/synapse/issues/17274, hopefully.

Basically, old versions of Synapse could advance streams without
persisting anything in the DB (fixed in #17229). On restart those
updates would get lost, and so the position of the stream would revert
to an older position. If this happened across an upgrade to a later
Synapse version which included #17215, then sync could get blocked
indefinitely (until the stream advanced to the position in the token).

We fix this by bounding the stream positions we'll wait for to the
maximum position of the underlying stream ID generator.