pgsql: Track statistics for spilling of changes from ReorderBuffer.

Track statistics for spilling of changes from ReorderBuffer.

This adds the statistics about transactions spilled to disk from
ReorderBuffer.  Users can query the pg_stat_replication view to check
these stats.

Author: Tomas Vondra, with bug-fixes and minor changes by Dilip Kumar
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/688b0b7f-2f6c-d827-c27b-216a8e3ea700@2ndquadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9290ad198b15d6b986b855d2a58d087a54777e87

Modified Files
--------------
doc/src/sgml/monitoring.sgml                    | 20 ++++++++++++
src/backend/catalog/system_views.sql            |  5 ++-
src/backend/replication/logical/reorderbuffer.c | 12 +++++++
src/backend/replication/walsender.c             | 42 +++++++++++++++++++++++--
src/include/catalog/catversion.h                |  2 +-
src/include/catalog/pg_proc.dat                 |  6 ++--
src/include/replication/reorderbuffer.h         | 11 +++++++
src/include/replication/walsender_private.h     |  5 +++
src/test/regress/expected/rules.out             |  7 +++--
9 files changed, 101 insertions(+), 9 deletions(-)