This patch adds a proc_name, proc_schema, hypertable_id index to
bgw_job. 3 functions using the new index are added as well:
ts_bgw_job_find_by_proc
ts_bgw_job_find_by_hypertable_id
ts_bgw_job_find_by_proc_and_hypertable_id
These functions are required for migrating the existing policies
to store their configuration in bgw_job directly.