Retry joining via other servers if first one failed. Fix some other bugs.

Add federation /version API

Raise 404 when couldn't find event

Don't include html comments in description

Fix backfill auth events

Check if we already have the events returned by /state/

Tweak integrity error recovery to work as intended

Change the way we summarize URLs

Retry event persistence on IntegrityError

Due to a bug in the porting script some backfilled events were not
correctly persisted, causing irrecoverable IntegrityErrors on future
attempts to persist those events.

This commit adds a retry mechanism invoked upon IntegrityError,
where when retried the tables are purged for all references to the
events being persisted.

Add /state_ids federation API

Port script: Handle the fact that some tables have negative rowid rows

keys/query: Omit device displayname if null

* Factor out more common code from the jenkins scripts

* Fix install_and_run path

* Poke jenkins

* Poke jenkins

... which makes it more consistent with user displaynames.