Fix query cache invalidation bug.

Fix query cache invalidation bug.

When an execute message is received, pgpool checks its max number of
rows paramter. If it's not zero, pgpool sets "partial_fetch" flag to
instruct pool_handle_query_cache() to not create query cache.  Problem
is, commit 2a99aa5d1 missed that even INSERT/UPDATE/DELETE sets the
execute message parameter to non 0 (mostly 1) and pgpool set the flag
for even none SELECTs. This resulted in failing to invalidate query
cache because if the flag is true, subsequent code in
pool_handle_query_cache() skips cache invalidation.  It was an
oversight in this commit (my fault):
https://git.postgresql.org/gitweb/?p=pgpool2.git;a=commit;h=2a99aa5d1910f1fd4855c8eb6751a26cbaa5e48d

To fix this change Execute() to check if the query is read only SELECT
before setting the flag.

Also add test to 006.memqcache.

Problem reported by and a test program provided by Achilleas Mantzios <a.mantzios@cloud.gatewaynet.com>.

Discussion: [pgpool-general: 9427] Clarification on query results cache visibility
https://www.pgpool.net/pipermail/pgpool-general/2025-April/009430.html

Backpatch-through: v4.2