v.io/x/ref/services/syncbase: fix Vanadium iterator usage

Vanadium iterators requires that before dropping them on the floor, either
Cancel() be called, or Advance() must have been called and returned false.
Without this, we risk leaks of resources incorporated in the iterators.

MultiPart: 1/2
Change-Id: I2eb14b62f23ce43720c352dcc541626583fe9f04
3 files changed