Set Isolation Level to READ_UNCOMMITTED for read-only datastore connection

[jqnatividad]

For large CKAN installations that use the Datastore and use postgresql replication, performance/concurrency has been vastly improved with the introduction of datastore.write_url and datastore.read_url set to the master and replica respectively.

Performance/concurrency can be further improved by allowing "dirty reads" of the datastore. This can be done by setting the SQLAlchemy engine isolation level to "READ_UNCOMMMITTED" for the read_url connection.

Installations that don't employ Postgres replication for the Datastore should also benefit from this change.