Review/rework of clock handling

The infos below are quite old (referencing Liquidsoap 1.4.3). Before doing any work, review clock handling of latest Liquidsoap versions which may differ fundamentally

• simultaneous streaming and recording from the same sound card input requires different clocks, otherwise the recording will be damaged in the event of a network failure
• https://www.liquidsoap.info/doc-1.4.3/clocks.html#external-clocks-decoupling-latencies