btr_page_tuple_smaller(): New function, refactored from
btr_page_split_and_insert().

btr_page_get_split_rec(): Renamed from btr_page_get_sure_split_rec().
Note that a NULL return may mean that the tuple is to be inserted into
either the lower or upper page, to be determined by btr_page_tuple_smaller().

btr_page_split_and_insert(): When btr_page_get_split_rec() returns NULL,
invoke btr_page_tuple_smaller() to determine which half-page the tuple
belongs to.

Reviewed by Sunny Bains