Before this change we used an overcomplicated method of memory
reservations in the pool.RW which caused deadlocks.
This changes it to use a much simpler reservation system where we
actually reserve the memory and store it in the pool.RW. This allows
us to use the semaphore.Weighted to count the actually memory in use
(rather than the memory in use and in the cache). This in turn allows
accurate use of the semaphore by users wanting memory.
Because multipart transfers can need more than one buffer to complete,
if transfers was set very high, it was possible for lots of multipart
transfers to start, grab fewer buffers than chunk size, then deadlock
because no more memory was available.
This fixes the problem by introducing a reservation system which the
multipart transfer uses to ensure it can reserve all the memory for
one chunk before starting.
