mempool: pick shard on every allocation; move types to pool_t

mempool: pick shard on every allocation; move types to pool_t

If we have a static choice of shard for every *type*
(pool_allocator_base_t) then we will hammer that shard from all
threads and play cacheline ping-pong.  Instead, move the types
list to pool_t (there aren't that many anyway) and pick a shard
on every allocate/deallocate call.

Signed-off-by: Sage Weil <sage@redhat.com>