Fix Entities.waitForServiceUp to avoid NPE

- Never return null for repeater’s `until`; always return TRUE or FALSE.
  Otherwise it will NPE.
Tidy EntityTransientCopyInternal
BrooklynLauncherTest: use LocalManagementContextForTests

- speeds up the test slightly
DynamicMultiGroup: fix+test removing empty bucket

- Previously gave ConcurrentModificationException when removing,
  iterating over Sets.difference, because that set is a live-view
  rather than a copy.
This closes #336