Rework ORTA/ORTV. Fixes #66, #64.

ORTA/ORTV can now be set to a number smaller than the number of MSTs,
leading to a=sendonly settings.
Also allowed VAD to work in CreateAnswer. (#64)
I don't think there's any case where that could happen. Here's a realistic case:

Originally offer of two audio streams, x and y:
m=audio X
a=msid:x
a=sendrecv
m=audio Y
a=msid:y
a=sendrecv

Then we stop one of the streams (x):
m=audio 0
m=audio Y
a=msid:y
a=sendrecv

Now we add two more audio streams, z and w. If ORTA is set to 3, we get:
m=audio Z
a=msid:z
a=sendrecv
m=audio Y
a=msid:y
a=sendrecv
m=audio W
a=msid:w
a=sendrecv

If ORTA is set to 2, we get:
m=audio Z
a=msid:z
a=sendrecv
m=audio Y
a=msid:y
a=sendrecv
m=audio W
a=msid:w
a=sendonly

And if ORTA is set to 1 (or less), we get:
If ORTA is set to 2, we get:
m=audio Z
a=msid:z
a=sendonly
m=audio Y
a=msid:y
a=sendrecv
m=audio W
a=msid:w
a=sendonly