Allow to have separate idle & silence thresholds

[skeller]

For asio (e.g. pipe) there is a difference between idle (no data being written to) and silence. Silence also happens on normal music (silent periods or fade out). Too short a silence timeout messes up playback with meta streams. The active stream would switch on fade-out / silence and immediately switch back on next track / non-silence, causing clients to resync and stutter.

This patch allows you to configure ilde_timeout independently from silence_timeout. Both options were undocumented for asio streams (pipe, pcm, ...). It also allows you to disable silence detection, getting back older snapcast behavior.