unique

- make each instance of ImportRemover have a unique propertyKey
  so multiple instances can work on the same set of nodes
- previous implementation was time based so collisions where possible
- uses AtomicInteger to be thread-safe

Change-Id: I271fb8ab6e1dd7236db486dc315bdf1184868f6f
Signed-off-by: Fabian Pfaff <fabian.pfaff@vogella.com>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.ui/+/183145
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Jeff Johnston <jjohnstn@redhat.com>