pgsql: Add wait events for WAL archive and recovery pause.

Add wait events for WAL archive and recovery pause.

This commit introduces new wait events BackupWaitWalArchive and
RecoveryPause. The former is reported while waiting for the WAL files
required for the backup to be successfully archived. The latter is
reported while waiting for recovery in pause state to be resumed.

Author: Fujii Masao
Reviewed-by: Michael Paquier, Atsushi Torikoshi, Robert Haas
Discussion: https://postgr.es/m/f0651f8c-9c96-9f29-0ff9-80414a15308a@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b8e20d6dabdafbe905b62910dc0236037c7881e1

Modified Files
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doc/src/sgml/monitoring.sgml      | 10 +++++++++-
src/backend/access/transam/xlog.c |  4 ++++
src/backend/postmaster/pgstat.c   |  6 ++++++
src/include/pgstat.h              |  4 +++-
4 files changed, 22 insertions(+), 2 deletions(-)