pgpool: Fix walker functions to return earlier.

Fix walker functions to return earlier.

Previously walker functions returned false once it found target
objects. But it is possible to short-circuit the traverse and it
should have been.

Also import comments from PostgreSQL to clarify walker function
design.

Author: Tatsuo Ishii <ishii(at)postgresql(dot)org>
Reported-by: liujinyang-highgo
Backpatch-through: v4.3
Discussion: https://github.com/pgpool/pgpool2/issues/143

Branch
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V4_3_STABLE

Details
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https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=64dda20f0070a6c64adb61433c07f89ea1c64244

Modified Files
--------------
src/rewrite/pool_timestamp.c | 40 +++++++++++++++++++++++++++++++++++++++-
src/utils/pool_select_walker.c | 18 +++++++++---------
2 files changed, 48 insertions(+), 10 deletions(-)