s3: use single memory pool
s3: use single memory pool

Previously we had a map of pools for different chunk sizes.
In practice the mapping is not very useful and requires a lock.
Pools of size other that ChunkSize can only happen when we have a huge file (over 10k * ChunkSize).
We need to have a bunch of identically sized huge files.
In such case most likely ChunkSize should be increased.

The mapping and its lock is replaced with a single initialised pool for ChunkSize, in other cases pool is allocated and freed on per file basis.

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone