ISPN-3718  Select which protobuf fields to index

Use protobuf schema doc comment annotations similar with the Hibernate Search annotations @Indexed and @Field.

https://issues.jboss.org/browse/ISPN-3718

depends on https://github.com/infinispan/protostream/pull/9 to be integrated first.
In theory, we could use a 'lighter' scheme than hibernate search:
Instead of:
```
@Indexed
message M {

   /**
    * @IndexedField(index=Index.YES, store=Store.NO)
    */
   optional int32 age = 6;
}
```

We could use:

```
message M {

   /**
    * @IndexedField(stored = true)
    */
   optional int32 age = 6;
}
```

Not sure if we need to mark messages as @Indexed, can it be inferred if any of the field is ```@IndexedField``` ?