[New feature] Force to make query cache.

[New feature] Force to make query cache.

Recognize /*FORCE QUERY CACHE*/ SQL statement comment so that any read
only SELECT/with queries are cached. This is opposite to /*NO QUERY
CACHE*/ comment. This feature should be used carefully. See the manual
for more details.

Discussion: https://github.com/pgpool/pgpool2/issues/56