Limit pinned memory pool chunk size

[vuule]

The size of chunks in the pinned memory pool should be limited by the slice size, since each chunk is always used by a single thread.

[madsbk]

I think this is a good idea but since it is possible to change the task_size (aka slice size) between IO calls, AllocRetain will have to support reuse of different sized memory chunks.

Also notice, there is only one memory pool shared between all threads. We could change this to have each thread maintain its own bounce buffer.