Refactor nbtree fastpath optimization.

Commit 2b272734, which added the fastpath rightmost leaf page cache
insert optimization, added code to _bt_doinsert() to handle using and
invalidating the backend local block cache.  It doesn't seem like a good
place to handle these low level details, though.  _bt_doinsert() is
supposed to be a high level function -- it is the main entry point to
nbtinsert.c.

Restructure the code by placing handling of the rightmost block cache at
the start of a new _bt_search() shim function, _bt_search_insert().  The
new function is called from _bt_doinsert(), which uses it as a
_bt_search() variant that conveniently accepts its BTInsertState state
as an argument.  _bt_doinsert() no longer needs to directly consider the
fastpath optimization.

Discussion: https://postgr.es/m/CAH2-Wzk59cxKJRd=rfbyub6-V4yWRjsOYRkUNHBLT1P1GdtCQQ@mail.gmail.com