[pgpool-committers: 5494] pgpool: Eliminate select(2) system calls when they are not necessary.

[Tatsuo Ishii]

Eliminate select(2) system calls when they are not necessary.

The idea is checking select(2) timeout parameter set in a static
variable in pool_read() and pool_read2(). If it's -1, it means no
select timeout will be set in pool_check_fd(), which implies we can
avoid to call pool_check_fd().

Also I moved pool_check_fd() and friends to pool_stream.c from a
modularity　point of view.

This gives slight performance improvement according to Jesper
Pedersen.

Bottleneck analysis and suggestions from Jesper Pedersen.
Discussion: [pgpool-hackers: 3247] https://www.pgpool.net/pipermail/pgpool-hackers/2019-February/003247.html

Branch
------
master

Details
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https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=cf5fe7eb89a11e3ada3833343c7ed6551ce7c662

Modified Files
--------------
src/include/pool.h                |   6 +-
src/include/utils/pool_stream.h   |   6 +-
src/protocol/pool_process_query.c |  94 +------------------------------
src/utils/pool_stream.c           | 115 +++++++++++++++++++++++++++++++++++++-
4 files changed, 119 insertions(+), 102 deletions(-)