Fix usage of generics in Infinispan Query

The DistributedExecutorMassIndexer class doesn't compile correctly according to JDT, the Eclipse compiler.
After initially suspecting that this was a bug of JDT - as it compiles just fine with javac from OpenJDK - I actually think that JDT has some good points, as the correctness can not be proven.

Not sure who's fooling me, but I could massage the code to make the types more explicit, and more importantly make all compilers happy.