Reduce path length for locking leaf B-tree pages during insertion

In our B-tree implementation appropriate leaf page for new tuple
insertion is acquired using _bt_search() function.  This function always
returns leaf page locked in shared mode.  In order to obtain exclusive
lock, caller have to relock the page.

This commit makes _bt_search() function lock leaf page immediately in
exclusive mode when needed.  That removes unnecessary relock and, in
turn reduces lock contention for B-tree leaf pages.  Our experiments
on multi-core systems showed acceleration up to 4.5 times in corner
case.

Discussion: https://postgr.es/m/CAPpHfduAMDFMNYTCN7VMBsFg_hsf0GqiqXnt%2BbSeaJworwFoig%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Yoshikazu Imai, Simon Riggs, Peter Geoghegan

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