srtpenc: Set the SRTP key as a shared key for all SSRCs

This is already how it was used.
srtpenc: Remove SSRC from signal, it's meaningless
srtpenc: Remove ssrc parsing from stream

It's no longer used
srtpenc: Remove useless structure

Remove now useless structure
srtpenc: Remove unused object members
srtpenc: Add config for the replay window size