improve performance of ThrottleStream a little bit
improve performance of ThrottleStream a little bit

* refs #111665
* save a syscall in most cases

Change-Id: Ibb0fc5991e16260b440b9493ad5a0ba9eabc727b
Signed-off-by: Kefu Chai <ef1142690fdf7c5d47bbd7e2a7a410478308cc21@mozy.com>
Reviewed-on: https://gerrit.dechocorp.com/59177
Reviewed-by: Jenkins <49695b0f9c673fd73dab0ff3f84b5ced9302cc6a@mozy.com>
Reviewed-by: Russ Simons <c93a72122f3199b175c9b2766a6096fa95d517d4@mozy.com>

mozy/mordor,mozy/mordor,adfin/mordor,mtanski/mordor,adfin/mordor,adfin/mordor,mtanski/mordor,mtanski/mordor,mozy/mordor