pgsql: Add pg_stat_archiver statistics view.

Add pg_stat_archiver statistics view.

This view shows the statistics about the WAL archiver process's activity.

Gabriele Bartolini, reviewed by Michael Paquier, refactored a bit by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/9132b189bf5589591cb63fef7952842b772e4fe6

Modified Files
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doc/src/sgml/monitoring.sgml | 67 +++++++++++++++++++
src/backend/catalog/system_views.sql | 11 ++++
src/backend/postmaster/pgarch.c | 21 +++---
src/backend/postmaster/pgstat.c | 121 ++++++++++++++++++++++++++++++++--
src/backend/utils/adt/pgstatfuncs.c | 69 +++++++++++++++++++
src/include/catalog/pg_proc.h | 2 +
src/include/pgstat.h | 34 +++++++++-
src/include/postmaster/pgarch.h | 13 ++++
src/test/regress/expected/rules.out | 8 +++
9 files changed, 327 insertions(+), 19 deletions(-)