Move the ringbuffer and counter clients to 'src/common/'

The clients are used by both liblttng-ust and liblttng-ust-ctl, now that
the clock and getcpu plugins on which they depend have been moved to
liblttng-ust-common, they can be extracted from liblttng-ust-ctl and
move to common convenience libraries.

Change-Id: I8384570007950ca18d00759790cb5264b7c3b528
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>