Wait for data availability when stopping a session

The lttng_stop_tracing now waits by default for data availability. A
status output is added in the library on stdout.

A no wait version is added to the API and the option no-wait is added to
the lttng command line for the stop command.

Also good to note that if a second stop_tracing call is done on an
already stopped session, the call will wait for data availability before
returning if the stop command on the session daemon returned an already
stopped error code.

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