debian: make sure the right pg version's binaries are in path
pgmemcache: stop using postgresql memory allocation in libmemcached

Libmemcached (and different versions of it) may or may not use the memory
allocation functions given to it with memcached_set_memory_allocators which
causes segmentation faults when calls to postgresql malloc/free are mixed
with calls to libc malloc/free.

It turns out that libmemcached calls from pgmemcache doesn't really need to
use postgresql's memory allocation functions as we never pass the data as-is
back to pg anyway, and we can just manually free the couple of temporary
allocations that are created.
pgmemcache: reorganize and clean up source code a bit

* Moved static function declarations to pgmemcache.c
* Fixed some minor indentation issues
* Fold do_memcache_set_cmd into memcache_set_cmd
tests: run memcached as a normal user so root access is not required
interval_to_time_t: handle span->day
Remove old servers from context when updating default_servers

libmemcached does not have a function for removing servers from the current
context so we must recreate the context from scratch when the list of
default servers changes.

The new functionality also makes it possible to implement a
memcache_server_delete() function at some point as requested in [#4] but
it's not implemented in this commit.