Fix confusion around single column statistics for leading column.
* They are handled as the first multi-column entry, not the first single-column entry. toString() thought it was the other way around.
* Shouldn't have a Generator for this column; it's handled by multi-column. The comment to this fact should have been an assert.
* Fix the multiplexing Visitor to not have that Generator which now gets the assert. It's a *huge* waste of space.
Distinguish row count and sampled count so that sampling is possible.
merge MMcM: Index statistics sampling groundwork

Merge pull request #885 from MMcM/akiban-server/index-statistics-sampling-groundwork
https://github.com/FoundationDB/sql-layer/pull/885