IncrementalIndex throws exception if limit exceeded

- For now uses a hardcoded ratio of aggregator to timeanddim buffer sizes
- canAppendRow is a workaround for realtime index since the
Firehose currently does not have a way of rolling back the last event in
case of error
- canAppendRow needs a fudge factor; there is a race between checking
if we can add a row and actually adding a row, because of the way MapDB
reports its size.