Add fast path for btree pages that fully satisfy query

[westonpace]

Currently we use the min/max of pages to determine which pages we need to search. Then we load those pages and search them with a flat search. Previously this was dominated by I/O time. Now that we are caching btree pages this flat search time could be a considerably part of the query.

We can speed this up by adding a fast path for pages that are obviously completely satisfied. For example, if the page has min=7 and max=11 and the query is foo BETWEEN 0 AND 20 then we know the page is completely matched and we can skip the flat search and just add all the row ids.