LTTng performance monitoring counters integration (work in progress)

LTTng performance monitoring counters integration (work in progress)

Integrate with Perf counters. Adds context fields to the events
recorded. Currently, cpu hotplug is still not supported, and the
callbacks are not called from the tracing code. It gives the rough idea
how to add context fields to events on either:

- per trace session
- per stream class
- per event type

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>