tmf: Handle race between state system queries and diposal

tmf: Handle race between state system queries and diposal

It's possible that a state system gets disposed while a query
is ongoing (happens between two successive .getNextChild()
calls, for example).

Instead of implementing a convoluted reader-writer-lock, we can
just catch the ClosedChannelException and rethrow a
StateSystemDisposed exception.

Change-Id: I93f72860bbb94a63e86e7c6f51682af659599fe3
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/8520
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>