Remove the internal memory allocator

Remove the internal memory allocator

The internal memory allocator had an extra code to keep a list of blocks
for small size allocation.  This would help to reduce the interactions
with the system malloc as the memory would be already allocated from the
system, but there's an extra cost associated with that - all the
allocations/deallocations must be locked, effectively eliminating any
optimizations in the system allocator targeted at multi-threaded
applications. While the isc_mem API is still using locks pretty heavily,
this is a first step into reducing the memory allocation/deallocation
contention.