Improve the jitter buffer

The idea is:
A new jitter buffer uses a timing histogram to compute the actual jitter relative to the audio device or system clock timer.
A bit sequence will be added to all transmitted frames to improve the jitter histogram computation.

Open issues:

• What about compatibility to old Jamulus servers and clients?
• How does the new auto jitter buffer algorithm behave? For what situation is it optimized? See, e.g., https://github.com/corrados/jamulus/issues/417.
• The old algorithm was designed to measure the block error rate for different jitter buffer sizes. What is the metric for the new auto jitter buffer algorithm to set the buffer size?
• We are using an unused bit in the OPUS bit stream. Question: Why is this bit not used? Is it a reserved bit? Do we have a reference to the OPUS specification document, where this bit is mentioned? Is there a risk that at a later version of OPUS that this bit is then used by OPUS and we cannot upgrade Jamulus to a new OPUS version?
• A documentation of the algorithm should be added so that others can understand what's going on and can improve/tune the new algorithm.

A first patch is already available: https://github.com/corrados/jamulus/pull/539.

See also the discussion in the pull request: https://github.com/corrados/jamulus/pull/529#issuecomment-678292903

首先审查 pull request #539 中的第一个补丁，以及 pull request #529 中的讨论。检查提议的时序直方图、传输帧的比特序列、与旧版客户端和服务器的兼容性，以及 OPUS 比特的使用情况。完成的条件是确定算法和指标，解决兼容性与规范相关的问题，并记录该算法。