The next commit that adds the kvdb to sql code migration will need to
initialize the accounts kvdb DB.
This commit exports that struct so that it can be referenced outside of
the accounts package.
We add a helper function to the functions that creates the test SQL
stores, in order to ensure that the store is properly closed when the
test is cleaned up.
Update the SQL migration to instead of using the public functions of the
BoltStore struct, we instead iterate over the buckets directly. This
ensures that the BoltStore struct Acccounts & LastIndexes functions of
the BoltStore implementation can be changed, without effecting the
SQL migration.
This commit introduces the migration logic for transitioning the
accounts store from kvdb to SQL.
Note that as of this commit, the migration is not yet triggered by any
production code, i.e. only tests execute the migration logic.
