PR: Zero copying in snapserver on TCP connections for audio data

[aanno]

This is the fix/enhancement for #1423

This is a zero copy implementation for TCP on linux.

I tackled the problem directly with sendmsg with MSG_ZEROCOPY and a dedicated thread for completion messages (MSG_ERRQUEUE). It's on an exclusive alternative path to the boost:asio stuff (as asio didn't support that) and it's linux only. But fallback is to the (alternative) asio implementation.

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