pgpool: Feature: allow to specify external command to retrieve replicat

Feature: allow to specify external command to retrieve replication delay.

This commit allows to specify an external command to retrieve
replication delay information for streaming replication standby nodes.
This feature is useful when backend database is a PostgreSQL
compatible third-party software which does not provide
pg_stat_replication view.

For this purpose a new GUC variable "replication_delay_source_cmd" is
added. The user provided command accepts replica node identifiers
(e.g, server1:5432 server2:5432), and prints replication delay amount
in seconds or milliseconds to stdout.

Also "replication_delay_source_timeout" GUC variable is added to give
up waiting for the command to finish if it takes long time.

Author: Nadav Shatz <nadav(at)tailorbrands(dot)com>
Reviewed-by: Tatsuo Ishii <ishii(at)postgresql(dot)org>
Discussion: https://www.postgresql.org/message-id/CACeKOO2E6cuCOQGFzq8i0%2BpFwi%3DJG4deiapHGkShjMjbn_-6tw%40mail.gmail.comxo

Branch
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master

Details
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https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=53844460fea156fbe1eb78a1adb87d65a8247b87

Modified Files
--------------
doc.ja/src/sgml/stream-check.sgml | 125 ++++++
doc/src/sgml/stream-check.sgml | 78 ++++
src/config/pool_config_variables.c | 23 +-
src/include/pool_config.h | 5 +-
src/sample/pgpool.conf.sample-stream | 14 +
src/streaming_replication/pool_worker_child.c | 461 ++++++++++++++++++++-
.../tests/041.external_replication_delay/README | 59 +++
.../tests/041.external_replication_delay/test.sh | 409 ++++++++++++++++++
.../041.external_replication_delay/test_parsing.sh | 54 +++
.../test_validation.sh | 323 +++++++++++++++
10 files changed, 1545 insertions(+), 6 deletions(-)