[Serusers] Call Forking and Mediaproxy

[Juha Heinanen]

reticent writes:

 > It seems to only allocates a single media session when use_media_proxy()
 > is called on a SIP message with multiple branches, so when one session
 > is established and the other is torn down, the media session with media
 > proxy is also torn down.

if invite forks and proxy can therefore receive several 1xx replies from
the branches, use_media_proxy() should recognize replies that belong
to the same session and update the state of this for each reply,
especially for final 2xx reply.

this is a known issue, but as far as i know, it has not been fixed yet.

- juha