Fix query cache bug reported in pgpool-general-jp:1441.

Fix query cache bug reported in pgpool-general-jp:1441.

In streaming replication mode with query cache enabled, SELECT hangs
in the following scenario:

1) a SELECT hits query cache and returns rows from the query cache.
2) following SELECT needs to search meta data and it hangs.

In #1, while returning the cached result, it misses to call
pool_unset_pending_response(), which leave the pending_response flag
be set. In #2, do_query() checks the flag and tries to read pending
response from backend. Since there's no pending data in backend, it
hangs in reading data from backend.

Fix is, just call pool_unset_pending_response() in #1 to reset the
flag.

Bug report and fix provided by Nobuyuki Nagai.

New regression test item (068) added by me.