[SPARK-4326] fix unidoc

There are two issues:

1. specifying guava 11.0.2 will cause hashInt not found in unidoc (any reason to force the version here?)
2. unidoc doesn't recognize static class defined in a base class

@aarondav @srowen @vanzin
IDE suggests that this is an unnecessary import, but it is necessary for unidoc. An alternative solution would be defined `Type` outside `BlockTransferMessage`.