Proto commits in sippy/rtpproxy

These 7 commits are when the Protocol Buffers files have changed:

Commit:ee1cfdc
Author:Maksym Sobolyev

o implement few remaining parts of the RFC4568 o optimize structure for fitting all the streams in the session into one request. We still need to do the reply part to allow multiple results to be returned.

The documentation is generated from this commit.

Commit:3f909e5
Author:Maksym Sobolyev

Implement rtpp_request.

Commit:c60b481
Author:Maksym Sobolyev

Add some comments clarifying use of some fields.

Commit:f297ad2
Author:Maksym Sobolyev

Add draft proto for the rtpproxy's responses.

Commit:797a9fd
Author:Maksym Sobolyev

sinfo -> stream_info.

Commit:4d07c04
Author:Maksym Sobolyev

Add (c).

Commit:e04e727
Author:Maksym Sobolyev

Add some sketch to implement SRTP control mechanism. Based largely on RFC4568 and follow-up ones.