Put meta index updates in the queue only if no similar updates are
already scheduled.

Change-Id: I529d30dcd8c2ce821e0f5381b46939973c541f0e
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.core/+/183142
Reviewed-by: Gayan Perera <gayanper@gmail.com>
Tested-by: Gayan Perera <gayanper@gmail.com>
Tested-by: JDT Bot <jdt-bot@eclipse.org>