tmf: Allow waiting for a state system's completion

tmf: Allow waiting for a state system's completion

ITmfStateSystem.waitUntilBuilt() will now block the calling
thread until the state system underneath is finished building.
This will be immediately useful for statistics (intermediate
queries are useless, we need to wait until it's ready anyway).

It's implemented using a CountDownLatch, similar to the
.waitForCompletion() in event requests.

Change-Id: I98420acc3ca2903d76a315259f25dca7d88b8d42
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/8427
Tested-by: Hudson CI