Remove the extra level of indirection via isc_memmethods_t

Remove the extra level of indirection via isc_memmethods_t

Previously, the applications using libisc would be able to override the
internal memory methods with own implementation.  This was no longer
possible, but the extra level of indirection was not removed.  This
commit removes the extra level of indirection for the memory methods and
the default_memalloc() and default_memfree().