This commit adds an `IteratorStats` that holds aggregate
iterator processing information. A method is also added to
`Iterator` to return the stats:
Stats() influxql.IteratorStats
The remote iterators will also emit their stats in the point
stream upon first connection, on a given interval, and then
finally once the last point has been sent.
This commit moves the `tsdb.Store.ExpandSources()` function onto
the `influxql.IteratorCreator` and provides support for issuing
source expansion across a cluster.
Jonathan A. Sternberg
Ben Johnson