dedicated async dispatcher scoped to graph queries

[ivanopagano]

The graph queries run on an implicit execution context configured through the akka config.
Since it's implicitly imported, now the context is available to other async operations in the world engine actor.
It would be good to encapsulate the implicit dispatcher in a module object that takes care to wrap and execute the queries in futures.

[ivanopagano]

refer to
http://stackoverflow.com/questions/17849749/passing-implicit-executioncontext-to-contained-objects-called-methods
for an interesting solution to this isolation problem