Permit use by non-superusers

Currently pg_shard requires superuser access for most activities, and all shards are created as owned by the superuser with superuser-only permissions. In order to make pg_shard work with DBAAS, we'll need to decouple superuser access requirements. It should require superuser to create the extension, but superuser access should not be required to create the shards, provided that the user has create table access on each shard database.