sliding sync: handle lone DELETE and INSERT operations
If you leave a room you can get a lone DELETE op.
If you join a room you can get a lone INSERT op.
Up until now, we've assumed these operations happen at the ends
of the list (e.g [0] or [length-1]) which is not guaranteed as it
depends on the sort order (e.g sort alphabetically and join a room
called 'D'). In this scenario, the indexes would not be tracked
correctly. Fixed with integration tests.
