Implement ALTER TABLE ... SPLIT PARTITION ... command

Implement ALTER TABLE ... SPLIT PARTITION ... command

This new DDL command splits a single partition into several partitions.  Just
like the ALTER TABLE ... MERGE PARTITIONS ... command, new partitions are
created using the createPartitionTable() function with the parent partition
as the template.

This commit comprises a quite naive implementation which works in a single
process and holds the ACCESS EXCLUSIVE LOCK on the parent table during all
the operations, including the tuple routing.  This is why the new DDL command
can't be recommended for large, partitioned tables under high load.  However,
this implementation comes in handy in certain cases, even as it is.  Also, it
could serve as a foundation for future implementations with less locking and
possibly parallelism.

Discussion: https://postgr.es/m/c73a1746-0cd0-6bdd-6b23-3ae0b7c0c582%40postgrespro.ru
Author: Dmitry Koval <[email protected]>
Co-authored-by: Alexander Korotkov <[email protected]>
Co-authored-by: Tender Wang <[email protected]>
Co-authored-by: Richard Guo <[email protected]>
Co-authored-by: Dagfinn Ilmari Mannsaker <[email protected]>
Co-authored-by: Fujii Masao <[email protected]>
Co-authored-by: Jian He <[email protected]>
Reviewed-by: Matthias van de Meent <[email protected]>
Reviewed-by: Laurenz Albe <[email protected]>
Reviewed-by: Zhihong Yu <[email protected]>
Reviewed-by: Justin Pryzby <[email protected]>
Reviewed-by: Alvaro Herrera <[email protected]>
Reviewed-by: Robert Haas <[email protected]>
Reviewed-by: Stephane Tachoires <[email protected]>
Reviewed-by: Jian He <[email protected]>
Reviewed-by: Alexander Korotkov <[email protected]>
Reviewed-by: Pavel Borisov <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Reviewed-by: Alexander Lakhin <[email protected]>
Reviewed-by: Kyotaro Horiguchi <[email protected]>
Reviewed-by: Daniel Gustafsson <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Reviewed-by: Noah Misch <[email protected]>