Fix broken longpoll/continuous _changes feed (regression)

In 013f88b I had changed the key prefix of channel-log documents, but I
forgot that the changeListener class hardcoded the old key prefix, so it
wouldn't recognize the new one.
DRYed up the code to avoid using the string "_sync:log2:" in more than
one place.
While I was at it, I did the same for "_sync:user:" and "_sync:role:".
Fixes #87 (again)