Session daemon health check support

Session daemon health check support

This is the first commit for the health check feature of the session
daemon.

Add a lttng_health_check(...) call to the public API and return 0 if
everything is fine or 1 if some health problem for a component was
detected.

Using this API call, you can either choose to test a specific component
such as the client command thread, the consumer thread(s), kernel
thread, application registration thread or all of them at the same time.

This feature is NOT implemented with the lttng command line UI and it is
intended to be like so until a stable version is accepted by the
community.

NOTE: The API could change so be aware of possible changes up to the
2.1-stable release.

Signed-off-by: David Goulet <dgoulet@efficios.com>