Add schedgraph traces for callout handlers. Specifically, a callwheel logs

Add schedgraph traces for callout handlers.  Specifically, a callwheel logs
a running event each time it executes a callout function.  The event
includes the function pointer, argument, and whether or not it was run from
hardware interrupt context.  The callwheel is marked idle when each handler
completes.  This effectively logs the duration of each callout routine in
the graph.