Let all the NewTestDB functions return the V1Store interface type
instead of pointers. Then add a manual skip in the TestGraphLoading test
for any non-bbolt backend. We can remove this once all the methods used
by the test have been implemented by the SQLStore. We only need the
manual skip for this one test since it is the only one that doesnt use
MakeGraphTest to init the graph db.
In this commit, we implement the postgres and sqlite versions of the
NewTestDB function. We add the various build flags so that only one of
the three versions of this function can be active at a time.
We also introduce the SQLStore struct which is the SQL implementation of
the V1Store interface.
NOTE: it currently temporarily embeds the KVStore struct so that we can
implement the V1Store interface incrementally. For any method not
implemented, things will fall back to the KVStore. This is ONLY the
case for the time being while this struct is purely used in unit tests
only. Once all the methods have been implemented, the KVStore field will
be removed from the SQLStore struct.
