Avoid to run out free query cache hash index entry.
Avoid to run out free query cache hash index entry.

If hash index entries are run out, pgpool-II cannot reuse old cache
entry because pgpool-II always expects there's at least one empty hash
index entry. To mitigate the problem, if free hash index entries are run
out, look for victim cache block to reuse the hash index entry.

Per bug #70.

treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres