Factor out the pre destination retry logic from TransactionQueue so it
can be reused in both get_pdu and crypto.keyring

This is so that the DeferredLists actually consume the error instead of
propogating down the non-existent errback chain. This should reduce the
number of unhandled errors we are seeing.

Federation min depth fix

This is to fix the issue where if a remote server sends an event
that references a really "old" event, then the local server will pull
that in and send to all clients.

We decide if an event is old if its depth is less than the minimum depth
of the room.

Hotfixes v0.7.0f

Application Services

Prepare the database whenever a connection is opened from the db_pool so...

Prepare the database whenever a connection is opened from the db_pool so that in-memory databases will work

When we see a difference in current state, actually use state conflict resolution algorithm