pgsql: Add pg_partition_root to display top-most parent of a partition

Add pg_partition_root to display top-most parent of a partition tree

This is useful when looking at partition trees with multiple layers, and
combined with pg_partition_tree, it provides the possibility to show up
an entire tree by just knowing one member at any level.

Author: Michael Paquier
Reviewed-by: Álvaro Herrera, Amit Langote
Discussion: https://postgr.es/m/20181207014015.GP2407@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3677a0b26bb2f3f72d16dc7fa6f34c305badacce

Modified Files
--------------
doc/src/sgml/func.sgml | 11 ++++
src/backend/utils/adt/partitionfuncs.c | 75 ++++++++++++++++++++++++----
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 5 ++
src/test/regress/expected/partition_info.out | 58 +++++++++++++++++++++
src/test/regress/sql/partition_info.sql | 13 +++++
6 files changed, 153 insertions(+), 11 deletions(-)