syncbase-java-highlevel: implement watch, plus a few other bits

other bits:
- replaces {add,remove}User with {invite,eject}User per
  feedback from Tao's API usability studies
- adds plumbing for managing watch change handles and
  syncgroup invite handles
- adds checks for syncgroup with no collections, or with
  collections whose blessings differ
- changes some TODOs to FIXMEs, to indicate priority

Change-Id: I3ca04358557f00ed3e25ef5b488fe30f1498614a
6 files changed