Fix MesosSupervisor.getMetadata

[erikdw]

This is intended to fix issue #119.

With the introduction of the TaskAssignments refactor for creating unique
task IDs (#106), I introduced a couple of bugs in the implementation of
MesosSupervisor.getMetadata:

1. The slot counts in the Storm UI were broken -- the return from
   getMetadata was always a single element vector, due to using a
   Set instead of a java array previously. This was causing the
   PersistentVector.create(Object ... object) method to be matched,
   which just puts the passed objects into a vector without iterating
   over their constituent elements. Since we are passing a single
   Set object, we are getting a single element in the resultant vector.
   So the fix is to just create a List and pass that to
   PersistentVector.create().
2. The returned Object must be serializable. Depending on the
   build and runtime environment, the serialization done by the storm
   supervisor during initialization will fail, crashing the supervisor.
   That was happening because we were passing back the
   ConcurrentHashMap$KeySetView object, which is not serializable.
   Here too, the fix is to just create a List and pass that to
   PersistentVector.create().

NOTE: I haven't been able to reproduce problem 2. unfortunately.

[drewrobb]

@erikdw this fixes problem 2 for me, thank you!