This replaces the `sync.Pool` allocator with lib/pool. This
implements a pool of buffers of up to 64MB which can be re-used but is
flushed every 5 seconds.
If `--use-mmap` is set then rclone will use mmap for memory
allocations which is much better at returning memory to the OS.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
