collapse qpdb implementation down to one file

collapse qpdb implementation down to one file

the code in qpdb.c was previously shared by qp-cachedb.c and
qp-zonedb.c.  since qp-zonedb.c no longer exists, it's not necessary
to keep these separate any longer. the two files have been merged,
and functions that were previously globally accessible have been
changed to static and renamed.