pgsql: Add new function pg_get_wal_summarizer_state().

Add new function pg_get_wal_summarizer_state().

This makes it possible to access information about the progress
of WAL summarization from SQL. The previously-added functions
pg_available_wal_summaries() and pg_wal_summary_contents() only
examine on-disk state, but this function exposes information from
the server's shared memory.

Discussion: http://postgr.es/m/CA+Tgmobvqqj-DW9F7uUzT-cQqs6wcVb-Xhs=w=hzJnXSE-kRGw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d9ef650fca7bc574586f4171cd929cfd5240326e

Modified Files
--------------
doc/src/sgml/func.sgml                 | 28 +++++++++++++++
src/backend/backup/walsummaryfuncs.c   | 39 ++++++++++++++++++++
src/backend/postmaster/walsummarizer.c | 65 ++++++++++++++++++++++++++++++++++
src/include/catalog/catversion.h       |  2 +-
src/include/catalog/pg_proc.dat        |  9 +++++
src/include/postmaster/walsummarizer.h |  4 +++
6 files changed, 146 insertions(+), 1 deletion(-)