Modify the way we benchmark mem_{get,put}

Modify the way we benchmark mem_{get,put}

Previously, the mem_{get,put} benchmark would pass the allocation size
as thread_create argument.  This has been now changed, so the allocation
size is stored and decremented (divided) in atomic variable and the
thread create routing is given a memory context.  This will allow to
write tests where each thread is given different memory context and do
the same for mempool benchmarking.