Split mux tracer into hot and cold parts

[coot]

Using non nullTracer has a quite big performance cost. We can split the mux
tracer into two parts, one that frequently logs (which would be turned off by
default) and one that is infrequent, and turned on by default.

Hot parts would be:

MuxTraceRecvHeaderStart -> Debug
MuxTraceRecvHeaderEnd {} -> Debug
MuxTraceRecvPayloadStart {} -> Debug
MuxTraceRecvPayloadEnd {} -> Debug
MuxTraceRecvStart {} -> Debug
MuxTraceRecvEnd {} -> Debug
MuxTraceSendStart {} -> Debug
MuxTraceSendEnd -> Debug
MuxTraceChannelRecvStart {} -> Debug
MuxTraceChannelRecvEnd {} -> Debug
MuxTraceChannelSendStart {} -> Debug
MuxTraceChannelSendEnd {} -> Debug
MuxTraceHandshakeStart -> Debug
MuxTraceHandshakeServerEnd -> Debug
MuxTraceRecvDeltaQObservation {} -> Debug
MuxTraceRecvDeltaQSample {} -> Debug

And the other one (cold):

MuxTraceState {} -> Info
MuxTraceCleanExit {} -> Notice
MuxTraceExceptionExit {} -> Notice
MuxTraceHandshakeClientEnd {} -> Info
MuxTraceHandshakeClientError {} -> Error
MuxTraceHandshakeServerError {} -> Error
MuxTraceSDUReadTimeoutException -> Notice

[coot]

Related to IntersectMBO/cardano-node#635 and IntersectMBO/cardano-node#636