audioringbuffer: parse channels field from compressed audio caps

Also parse channels as an optional field in the caps for compressed
audio formats.
adder: rework the logic to check if eos has to be sent.

Checking the size available was incorrect, and the infos
for per-pad EOS are available.

Same logic as audiomixer.

fixes: https://bugzilla.gnome.org/show_bug.cgi?id=727025
sdp: guard against address parse errors.
decodebin: In adaptive streaming mode, only have a fixed buffer limit for the non-buffering multiqueue