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
--------------
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(-)