nbtree: Avoid allocating _bt_search stack.

nbtree: Avoid allocating _bt_search stack.

Avoid allocating memory for an nbtree descent stack during index scans.
We only require a descent stack during inserts, when it is used to
determine where to insert a new pivot tuple/downlink into the target
leaf page's parent page in the event of a page split.  (Page deletion's
first phase also performs a _bt_search that requires a descent stack.)

This optimization improves performance by minimizing palloc churn.  It
speeds up index scans that call _bt_search frequently/descend the index
many times, especially when the cost of scanning the index dominates
(e.g., with index-only skip scans).  Testing has shown that the
underlying issue causes performance problems for an upcoming patch that
will replace btgettuple with a new btgetbatch interface to enable I/O
prefetching.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/CAH2-Wzmy7NMba9k8m_VZ-XNDZJEUQBU8TeLEeL960-rAKb-+tQ@mail.gmail.com