Update SipUserAgent.h

[skopenko]

Change to comply RFC

[skopenko]

T1 500 ms default round-trip time (RTT) estimate
T2 4 s The maximum retransmit interval for non-INVITE
requests and INVITE responses
T4 5 s Maximum duration a message will remain in the
network

[joegen]

Thanks for the pull request. However, we have reasons why those values are lower than the RFC recommendation of 500 ms for timer T1. We use this faster timer interval to make sure that failover happens faster than it would as the RFC mandated. If you want this pull request to be approved, you need to make this a configurable parameter with 100 ms as the default.