Add support for the m.replace and m.reference bundled
aggregations.
This should fix plenty of subtle client issues.
Threads are not included in the new code as they have
historically been written to the database. Replacing the
old system would result in issues when switching away from
continuwuity, so saved for later.
Some TODOs have been left re event visibility and ignored users.
These should be OK for now, though.
Also fixes:
- Transaction IDs leaking in event route
- Age not being set for event relations or threads
- Both of the above for search results
Notes down concern with relations table
