Set Isolation Level to READ_UNCOMMITTED for read-only datastore connection

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.

The issue names datastore.read_url as the entry point; inspect how that read-only connection is configured and where its isolation level is set. Done means the read_url connection uses READ_UNCOMMITTED while installations without PostgreSQL replication continue to benefit, with relevant behavior covered by tests.

Written by the indexing model from the issue text.