tmf: Synchronize accesses to HistoryTree's latest branch

tmf: Synchronize accesses to HistoryTree's latest branch

Most accesses are punctual, so the synchronizedList wrapper covers
for most of them. There is the iteration in readNode() that is done
by reader threads, but it's usually very quick, so there was not
really much advantage of using a ReadWriteLock - considering the
downside of more verbose locking.

Also made the list final and avoided exposing it publicly - the
backend object only needs the root node.

Change-Id: I46c315d46a5e96d609c052d4cedb4404fe765176
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/21219
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
IP-Clean: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Hudson CI