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.
This commit adds the SQL implementation of the session.Store interface.
This can be run against all session unit tests via `make unit
pkg=session tags=test_db_sqlite` and `make unit pkg=session
tags=test_db_postgres`.
When we link a session to an account, we want to be able to validate
that the account actually does exist. So in preparation for that, we
first give the session store access to the accounts store.
In preparation for adding helpers with the same names but that will
compile under different build flags, we add the helper DB constructors
to use when testing the session store logic against a KVDB backend.
