dynamic buffer pool, used for server TCP for the moment

[aanno]

I did some profiling on snapcast (while working on #1423). From my measurements, I concluded that a dynamic buffer pool for audio data would increase the performance.

Hence this is such a dynamic buffer pool. It is at common/buffer_pool.* that makes it possible to use it in server and client. However, this PR only uses it for TCP in the server (for shared_const_buffer). But I guess it could be used in the client for for WS as well.

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