Consider moving some of the Rows methods to a new type

[flimzy]

Some of the Rows methods return data this is only available after iterating the entire result set. This has lead to several bug reports, because it's not very intuitive.

Perhaps a new method can be added that returns these values while closing the iterator. Perhaps `Finish() would be a good name?

// ResultMetadata contains metadata about certain queries.
type ResultMetadata struct {
    Offset int64
    TotalRows int64
    UpdateSeq string
    Warning string
    QueryIndex int
    Bookmark string
}

// Finish iterates through any remaining items in the iterator, and collects summary metadata, then closes the iterator.
func (*Rows) Finish() (ResultMetadata, error)