We discovered that class was registering a `finalClose` to be
executed on reactor shutdown.

On the multiuser scenario, a logout destroys Soledad and should
properly terminate everything related to it. That SQLCipherU1DBSync
instance was being held even after logout by the reactor so it
could call that `finalClose` on shutdown.

The `finalClose` only set running to False and set a `shutdownID` that
was not used anywhere else, so we removed it and moved setting
running to False to the `close` function method. That way we preserve
the functionality but let the instance be properly garbage collected
on logout.

Otherwise it will put the exception as an additional parameter.

this is needed for some mail tests.

This was discovered during load tests: Trying to process more than 999
docs triggers an error on SQLite due a select query not supporting 999
values to query.

test_processing_order aims to check that unordered docs wont be
processed, but if we let the pool start and advance Twisted LoopingCall
clock right before calling the processing method manually, the process
method will run concurrently and cause a race condition issue.