[reSIProcate] Message::brief

[david Butcher]

I refactored the way Message::brief() works. It no longer uses the heap.

Applications that have classes that derive from Message or ApplicationMessage
will need to rewrite virtual Data brief() const as virtual std::ostream&
encodeBrief(std::ostream&) const.

Use of brief() should be largely backwards compatible. If not, you can use
resip::Data::from(foo.brief()) where you used foo.brief().

david