After we receive one document from the target database, we have to update the
target metadata or else we will not be able to succesfully return the new
generation and transaction id of the target when receiving exactly one
document during a sync.

Tests actually expect a tuple instead of a list on the return value of
get_sync_info().

Previous to this modification, the initialization of the sync decrypter pool
could happen concurrently with other database operations. That could cause the
pool to hang because it could be waiting for something that was mistakenly
deleted because of the wrong order of database operations.

This commit implements a standard which we already use in leap.keymanager and
leap.mail which makes some methods wait for the initialization operation
before they are actually called.

Closes: #7386

The old strategy for having Soledad sync running asynchronously with other
API calls was to have the sync running in its own threadpool. This is not
needed now that all sync code uses deferreds and will not block. This commit
removes what's left from the old threadpool.

Previous to this modification, the post-sync hooks were run after the sync
lock was released. In that scenario, the next sync process could start before
the previous sync's post-sync hooks were run. In general, we want the hooks to
run while the current sync lock is still locked, even though for some plugins
this might not make a difference.