New port: databases/pg_partman

pg_partman is an extension to create and manage both time-based and
serial-based table partition sets. Sub-partitoning is also supported.
Child table & trigger function creation is all managed by the extension
itself. Tables with existing data can also have their data partitioned in
easily managed smaller batches. Optional retention policy can automatically
drop partitions no longer needed. A background worker (BGW) process is
included to automatically run partition maintenance without the need of an
external scheduler (cron, etc) in most cases.

WWW: https://github.com/keithf4/pg_partman

PR: 214417
Submitted by: Lacey Powers <lacey.leanne@gmail.com>
Reviewed by: ler