Reintroduce `withConnection` and fix provisioning

This restores the pre-1.0.8 behavior of automatically establishing a connection to Cassandra. `Adapter#provision` now behaves as expected (the `DESCRIBE` command is not exposed to CQL, but it is possible to obtain a list of available keyspaces from the `system.schema_keyspaces` meta-table); to that end, the `noprovision` option has been removed.