Fix leaf-page only search and search-near operations

WT-3769 broke leaf-page only searches.

The WT_CURSOR.search and WT_CURSOR.search_near operations are supposed to search any pinned Btree leaf page (that is, any page pinned as a result of a previous operation), assuming locality of reference, that a search is likely to hit on the currently pinned page. Commit 00e1b59 broke this by checking if the current key was internal (referencing an on-page key) or external (referencing a key set by the application using WT_CURSOR.set_key). That test was correct for non-search operations (for example, WT_CURSOR.update, but not for search operations).