fs/asyncreader: optionally user mmap for memory allocation with --use-mmap #2200

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.

fs/config.go

@@ -86,6 +86,7 @@ type ConfigInfo struct {
 	StatsOneLine          bool
 	Progress              bool
 	Cookie                bool
+	UseMmap               bool
 }
 
 // NewConfig creates a new config with everything set to the default