Fix limit logic for EventsStream (#7358)

* Factor out functions for injecting events into database

I want to add some more flexibility to the tools for injecting events into the
database, and I don't want to clutter up HomeserverTestCase with them, so let's
factor them out to a new file.

* Rework TestReplicationDataHandler

This wasn't very easy to work with: the mock wrapping was largely superfluous,
and it's useful to be able to inspect the received rows, and clear out the
received list.

* Fix AssertionErrors being thrown by EventsStream

Part of the problem was that there was an off-by-one error in the assertion,
but also the limit logic was too simple. Fix it all up and add some tests.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/7358.bugfix b/changelog.d/7358.bugfix
new file mode 100644
index 0000000000..f49c600173
--- /dev/null
+++ b/changelog.d/7358.bugfix
@@ -0,0 +1 @@
+Fix a bug where event updates might not be sent over replication to worker processes after the stream falls behind.