Introduced IParallelizable interface to mark search participants,
patterns and scopes eligible for parallel search.

If search scope, participant and pattern given to PatternSearchJob all
support parallel search, parallelize index search by using
ForkJoinPool.commonPool() to schedule search tasks.

If this behavior is not desired,
org.eclipse.jdt.core/enableParallelJavaIndexSearch=false preference can
be set.

Change-Id: Iad18e2d14a9ce2603e5704197706c97bbfb8eea9
Signed-off-by: Gayan Perera <gayanper@gmail.com>