slab_allocator: remove SlabAllocator

Now that Buffer instantiation has improved, the SlabAllocator is an
unnecessary layer of complexity preventing further performance
optimizations.

Currently there is a small performance loss with very small stream
requests, but this will soon be addressed.
smalloc: use malloc instead of new

To use realloc the implementation had to be changed to use malloc/free.
{stream,udp,tls}_wrap: remove unused offset/length

The function arguments offset and length are now no longer used since
all I/O requests now use discretely allocated memory.