Move the SDP offer/answer model to the sdp_create
approach instead of using the sdp_replace one.
This assumes the SDP body including session level
attributes (s=, o=, t= etc.), as well as the media
attributes, are formatted using currently given
session context.
In other words, rtpengine collects all possible
information during all of the offer/answer exchanges
within the dialog, which in its turn affects each
monologue's context, from which each new SDP message
will be built up.
This approach replaces the older one, which instead
used to go through the currenty processed SDP and
just replace those attributes, that rtpengine
is required to affect, leaving the rest untouched.
Additionally: all of the existing offer/asnwer model
tests were fixed to comply with the currect change.
Most of it is just an offset of attributes, which are
stored a bit higher/lower within the same media or global
SDP session.
Also a good part of unit test fixes targets a move of
`c=` line from the session level to the media one (so
each media has from now on its own `c=` attribute). This
does discontinue a support for session level connection
information.
Change-Id: Iecb4739683d23c4f9341e8a34b71f8ca2070956c