Fix pair state for 'unsynchronized' and 'synchronized'

Found by Sentry, sqlite.py:L893 :

Unknown pair state for 'unsynchronized' and 'synchronized'

A document local state is set as unsynchronized to ignore it.
Here, when updating the document, we set its remote state as synchronized because it has the right digest.
Problem is we haven't assigned a pair state for (unsynchronized, synchronized).

We need to add this pair state.