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/configflags/configflags.go

@@ -88,6 +88,7 @@ func AddFlags(flagSet *pflag.FlagSet) {
 	flags.BoolVarP(flagSet, &fs.Config.StatsOneLine, "stats-one-line", "", fs.Config.StatsOneLine, "Make the stats fit on one line.")
 	flags.BoolVarP(flagSet, &fs.Config.Progress, "progress", "P", fs.Config.Progress, "Show progress during transfer.")
 	flags.BoolVarP(flagSet, &fs.Config.Cookie, "use-cookies", "", fs.Config.Cookie, "Enable session cookiejar.")
+	flags.BoolVarP(flagSet, &fs.Config.UseMmap, "use-mmap", "", fs.Config.UseMmap, "Use mmap allocator (see docs).")
 }
 
 // SetFlags converts any flags into config which weren't straight foward