Add current prio to proc state, track prio changes

Add current prio to proc state, track prio changes

This adds a prio field to the Process class from linuxautomaton, which
contains the process' current priority value. On priority change, a
notification is sent, allowing analyses to track priority
changes. Currently, the sched analysis tracks these priority changes
via a per-process PrioEvent list, formed by (timestamp, prio) tuples.

Signed-off-by: Antoine Busque <abusque@efficios.com>