Fix FindBugs warnings in sal-remoterpc-connector and enable enforcement

Fix FindBugs warnings in sal-remoterpc-connector and enable enforcement

Warnings fixed:
  - RemoteRpcImplementation: use of 'error' known to be null
  - RpcBroker, RpcRegistry: The Creator class has non-Serializable field.
    Removed the Creator class and used Props that creates by reflection.
  - RpcBroker: use of 'result' that is marked as @Nullable
  - RpcBroker: redundant check of 'result.getErrors()' that is known to be
    non-null (marked as @Nonnull).
  - Gossiper, RemoteRpcRegistryMXBeanImpl: use entrySet iterator instead of
    keySet and get.
  - Messages: redundant specification of implements Serializable
  - LatestEntryRoutingLogic: Comparator should also implement Serializable in
    case TreeSet is serialized. This isn't the case here but it doesn't hurt
    to implement Serializable in lieu of supressing the warning.
  - LatestEntryRoutingLogic: Fixed potential null pointer de-reference in
    'compare'.

Change-Id: I8930c8975e1dd9179d78e74087b3994a365b90f8
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>