sched: add new timeout class for peer transmissions

This allows transmissions in symmetric mode to be scheduled
independently from client transmissions. This reduces maximum delay
in scheduling when chronyd is configured with a larger number of
servers.

sched.h

@@ -34,7 +34,8 @@ typedef unsigned int SCH_TimeoutID;
 
 typedef enum {
   SCH_ReservedTimeoutValue = 0,
-  SCH_NtpSamplingClass,
+  SCH_NtpClientClass,
+  SCH_NtpPeerClass,
   SCH_NtpBroadcastClass,
   SCH_NumberOfClasses /* needs to be last */
 } SCH_TimeoutClass;