Additional empty-value support: add a new page-header flag if all of the
values are empty, we anticipate data sets where that will be the case.

This also fixes a mistake in the last set of changes, we weren't setting
the no-empty-values flag in the case of a bulk load.
Merge pull request #838 from wiredtiger/empty-value-handling

Additional empty-value support: add a new page-header flag if all of the values are empty.