Smaller PTIME for L16 codecs

[bennylp]

Currently the PTIME for L16 codecs are set to either 10 or 20 ms. There are few problems with this:

1. packet may be too large for the MTU
2. we will not be able to receive incoming frames with size smaller than our PTIME. This can happen because theoretically one L16 frame can consist of as few as 1 sample.

Some changes that can be done to fix this:

1. Give the ability for the application to specify PTIME on per L16 codec basis.
2. Add buffering in the codec. So while the PTIME settings is mandatory in pjmedia framework and it will be used to control the size of outgoing packet, the codec should be able to receive frames with less than PTIME samples. And it does this by buffering the incoming frame if it has less than PTIME samples.

[nanang]

TeluuCon? 2018: no urgency.