change Set -> Vector in CommandCompletionsResponse

Inside the protocol, we should avoid:
1. abstract collection types (the concrete type may or may not be
supported by pickling)
2. Sets in general since it adds non-determinism.
Initial compilation with play-json instead of pickling

 - this is full of unimplemented ??? and doesn't work,
   it does mostly compile
 - integration tests project appears to hang forever when
   compiling
 - introduces sbt.serialization package
 - should cleanup EventTracker.scala removing EventWithWrites
 - see other TODO and FIXME