pgsql: Expose some information about backend subxact status.

Expose some information about backend subxact status.

A new function pg_stat_get_backend_subxact() can be used to get
information about the number of subtransactions in the cache of
a particular backend and whether that cache has overflowed. This
can be useful for tracking down performance problems that can
result from overflowed snapshots.

Dilip Kumar, reviewed by Zhihong Yu, Nikolay Samokhvalov,
Justin Pryzby, Nathan Bossart, Ashutosh Sharma, Julien
Rouhaud. Additional design comments from Andres Freund,
Tom Lane, Bruce Momjian, and David G. Johnston.

Discussion: http://postgr.es/m/CAFiTN-ut0uwkRJDQJeDPXpVyTWD46m3gt3JDToE02hTfONEN=Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/10ea0f924a2788f9e701d6213745aaa5ca3efb8a

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 18 ++++++++++++++
src/backend/storage/ipc/sinvaladt.c | 13 +++++++---
src/backend/utils/activity/backend_status.c | 4 ++-
src/backend/utils/adt/pgstatfuncs.c | 38 +++++++++++++++++++++++++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 7 ++++++
src/include/storage/sinvaladt.h | 4 ++-
src/include/utils/backend_status.h | 11 +++++++++
8 files changed, 90 insertions(+), 7 deletions(-)