FDN-2460: Patch partman5.run_maintenance to avoid table scans (#29)

The real diff this introduces is https://github.com/pgpartman/pg_partman/pull/662/files.

Avoiding table scans helps making `run_maintenance` fast, which is important because it takes an exclusive lock which prevents other uses of the table that it's processing. (The exclusive lock is acquired when dropping partitions to enforce the retention policy.)

More background in [FDN-2442].

Author: nicolagi