Aggregator to get exact sums of counter requests

Modifications to iterators to disable interpolation optionally.  In addition, a new aggregation function, ESUM ("explicit sum") that disables interpolation and can be used to get exact sums of only the explicitly stored data points in a time range.  This is a requirement for metering applications in which you want to be able to get back an exact sum of all the counts that have been stored in the database over that interval across all applicable tags.

This is addressing the problem described in issue #55 (https://github.com/OpenTSDB/opentsdb/issues/55#issuecomment-3083647)