Protected by FLAGS_enable_quic_stateless_reject_support. Add support

Protected by FLAGS_enable_quic_stateless_reject_support.  Add support
for stateless rejects to the QuicDispatcher code.

QuicServerSession - Added extra setter, which the dispatcher uses to
indicate that session should issue stateless rejects (if supported by
the peer).

QuicDispatcher - Will use stateless rejects if the session map is over
a threshold, configurable via flag.  If a stateless reject has been
issued, the dispatcher will close the session silently.  It will also
add the connection-id to the time-wait list, indicating that further
packets for the connection-id should be black-holed.

Changes to the end_to_end_tests will be made in a separate CL.

Merge internal change: 93414515

R=rch@chromium.org

[Ryan Hamilton, 2015-05-15 04:02:39]

lgtm