In preparation for upcoming migration tests from a kvdb to an SQL store,
this commit updates the NewTestDB function to return the Store interface
rather than a concrete store implementation.
This change ensures that migration tests can call NewTestDB under any
build tag while receiving a consistent return type.
In this commit, we add the SQLStore type which implements the
accounts.Store interface. To demonstrate that it works as expected, we
also plug this implementation into all the account unit tests to show
that they pass against the sqlite and postgres backends.
One can use `make unit pkg=accounts tags=test_db_postgres` or
`make unit pkg=accounts tags=test_db_sqlite` to test locally.
Note that 2 small timestamp related changes are made to the unit tests.
This is to compensate for timestamp precision in postgres.
