Support receiving Opus packets with various frame lengths

[ming]

From the ​RFC 7587:

The Opus encoder can output encoded frames representing 2.5, 5, 10,
   20, 40, or 60 ms of speech or audio data.  Further, an arbitrary
   number of frames can be combined into a packet, up to a maximum
   packet duration representing 120 ms of speech or audio data.

For example, for a packet duration of 40ms, we can receive 1 frame of 40ms, 2x20ms, 4x10ms, etc. However, currently pjmedia only expects constant frame length to be specified during stream creation.

Thank you to Marcus Froeschl for the suggestion and patch testing.

[ming]

In 5734:

Fixed #2089: Support receiving Opus packets with various frame lengths

[nanang]

In 5799:

Re #2089:

• Set default max Opus RX frame length, i.e: param.info.max_rx_frame_size, to 1275. This is to avoid issue of truncated frame in jitter buffer when incoming frame length gets increased (after remote Opus encoder changes Opus params on the fly, e.g: ptime, VBR/CBR, sampling rate). Thanks Marcus Froeschl for the report, the investigation, and the solution.
• Fixed pjmedia_codec_opus_set_default_param() to initiate codec param with current default settings.
• Fixed some compile warnings.

[ming]

Replying to nanang:

In 5799:

Re #2089:

• Set default max Opus RX frame length, i.e: param.info.max_rx_frame_size, to 1275. This is to avoid issue of truncated frame in jitter buffer when incoming frame length gets increased (after remote Opus encoder changes Opus params on the fly, e.g: ptime, VBR/CBR, sampling rate). Thanks Marcus Froeschl for the report, the investigation, and the solution.
• Fixed pjmedia_codec_opus_set_default_param() to initiate codec param with current default settings.
• Fixed some compile warnings.

The initialization of codec param (the second fix above) is reverted in r5831, since it will overwrite user's supplied codec param.