loadHorizon and unloadHorizon by record count

[flexyford]

The loadHorizon and unloadHorizon should determine the number of records to load, not the number of pages to load.

loadHorizon: If some records in a page lie inside the loadHorizon, then load that page.

unloadHorizon: If all the records in a page lie outside the unloadHorizon, then unload that page.

[cowboyd]

unloadHorizon: If the unload Horizon extends into a page, unload the page that follows.

or that preceeds? Perhaps a better statement is "if all the records in a page lie outside the unloadHorizon, then unload that page.

[flexyford]

Edited description to match your comments

[flexyford]

Fixed on #14