[reSIProcate] Via Branches: format change, cookie change.

[Adam Roach]

As part of the SigComp integration, I updated the Via header field 
branch parameter, since I need to squirrel away information on a 
per-transaction basis, and I need to get to it down in the transports 
(before transaction correlation is performed).

Scott Godin discovered that this change leads to incompatibilities when 
running an older resip client against a newer resip proxy. I've fixed 
this problem by changing the resip cookie from -d87543- to -d8754z-.

I don't suspect anyone really cares about this change, but it seemed 
fundamental enough that I wanted to put it on the list so that any 
objections can be raised.

/a