aubuf: better support for different put/get ptime

[cspiel1]

By default the ptime for writing frames to the aubuf is 20ms. If e.g. module
pulse_async is used the ptime for reading dynamically changes and lies around
5ms.

• Compute ptime for get/put separately, and consider channel number.
• Computed jitter was too low. Reset ts0 and tr0 only if a frame is missing.
• Support wish size values that are not a multiple of written frames, if read
  frames are smaller. This avoids underruns at the beginning of a stream.
  E.g.: 20ms frames are written, 5 ms are read and minimum size is set to 30ms.
  Then two calls to aubuf_get() after there are 20ms in the aubuf still return
  silence frames. The thirds aubuf_get() returns the first 5ms frame.
• Remove the +1 byte for max_sz. This makes no sense and no difference.

Tested with:

• pulse and pulse_async
• opus, g711, g722
• different aubuf min size
• with and without jitter simulation

Here a plot with pulse_async, opus and 30ms audio buffer. The account has ptime=20ms which is default. For the pulseaudio stream minreq=5ms.