* Split state group persist into seperate storage func
* Add per database engine code for state group id gen
* Move store_state_group to StateReadStore
This allows other workers to use it, and so resolve state.
* Hook up store_state_group
* Fix tests
* Rename _store_mult_state_groups_txn
* Rename StateGroupReadStore
* Remove redundant _have_persisted_state_group_txn
* Update comments
* Comment compute_event_context
* Set start val for state_group_id_seq
... otherwise we try to recreate old state groups
* Update comments
* Don't store state for outliers
* Update comment
* Update docstring as state groups are ints
1. use `deferred.errback()` instead of `deferred.errback(e)`, which means that
a Failure object will be constructed using the current exception state,
*including* its stack trace - so the stack trace is saved in the Failure,
leading to better exception reports.
2. Set `consumeErrors=True` on the ObservableDeferred, because we know that
there will always be at least one observer - which avoids a spurious "CRITICAL:
unhandled exception in Deferred" error in the logs
ObserveableDeferred expects its callbacks to be called without any
logcontexts, whereas it turns out we were calling them with the logcontext of
the request which initiated the persistence loop.
It seems wrong that we are attributing work done in the persistence loop to the
request that happened to initiate it, so let's solve this by dropping the
logcontext for it.
(I'm not sure this actually causes any real problems other than messages in the
debug log, but let's clean it up anyway)
Add db_conn parameters to the `__init__` methods of the *Store classes, so that
they are all consistent, which makes the multiple inheritance work correctly
(and so that we can later extract mixins which can be used in the slavedstores)
This is due to the fact that we prefilled caches using txn.call_after,
which always gets called including on error.
We fix this by making txn.call_after only fire when a transaction
completes successfully, which is what we want most of the time anyway.
