Three STUN binding request properties should be customisable. RFC 5245
describes the retransmission timer of the STUN transaction 'RTO', and
RFC 5389 describes the number of retransmissions to send until a
response is received 'Rc'. The third property is the 'RTO' when
a reliable connection is used.

RFC 5389 introduces a supplementary property 'Rm' as a multiplier used
to compute the final timeout RTO * Rm. However, this property is not
added in libnice, because this would require breaking the public API for
STUN. Currently, our STUN implementation hardcodes a division by two for
this final timeout.

Differential Revision: https://phabricator.freedesktop.org/D1109