common: Concurrency fixes in TraceCompassActivator

common: Concurrency fixes in TraceCompassActivator

The TraceCompassActivator class (which can be used as base for any
Activator in every plugin) tracks all the loaded activators in a map.

Add a bit of synchronization to make sure this map cannot be victim
of unexpected concurrent accesses.

Change-Id: Ia94899dfb140a5bf740e3cc6343f6d43579dd777
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/50752
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>