btr_cur_t::search_leaf(): Invoke btr_cur_need_opposite_intention() after
positioning page_cur.rec so that the record will be in the intended page.
This is something that was broken in
commit f2096478 or
commit de4030e4 or related changes.

btr_cur_need_opposite_intention(): Add a debug assertion that would
catch the misuse.

The "next line of defence" that should have caught this bug in debug builds
are assertions that mtr_t::m_memo contains MTR_MEMO_X_LOCK for the
dict_index_t::lock. When btr_cur_need_opposite_intention() holds,
we should escalate to acquiring an exclusive index->lock in
btr_cur_t::pessimistic_search_leaf().

Reviewed by: Debarun Banerjee