Properly use the per process APR memory pool
Properly use the per process APR memory pool

Previously an ephemeral memory pool was being assigned to
`mapcache_context->process_pool` where instead a process peristent
memory pool should have been used. This manifested itself when trying
to use the BDB cache storage instead of tiles.

geo-data/node-mapcache,geo-data/node-mapcache,geo-data/node-mapcache