v0.0.13: Implement lock-free DB transactions.

Lock-free write transactions allows GraphKB to have multiple instances
running in parallel to scale out.

To implement the feature I added the source breakdown in 'assets' and
'relations' tables. That way there is no concurrent access to pivot
points anymore since each data source produces and manage its own entry
in the DB. The reconciliation is done at query time once the entries are
retrieved from DB.