vpnkit-forwarder: avoid reading headers 2 bytes at a time.
vpnkit-forwarder: avoid reading headers 2 bytes at a time.

Add a layer of buffering to the Reader so the field-by-field reading
doesn't result in a lot of system calls.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit