Expire sliding sync connections (#19211)

We add some logic to expire sliding sync connections if they get old or
if there is too much pending data to return.

The values of the constants are picked fairly arbitrarily, these are
currently:
1. More than 100 rooms with pending events if the connection hasn't been
used in over an hour
2. The connection hasn't been used for over a week

Reviewable commit-by-commit

---------

Co-authored-by: Eric Eastwood <erice@element.io>

changelog.d/19211.misc

@@ -0,0 +1 @@
+Expire sliding sync connections that are too old or have too much pending data.