Further _bt_first simplifications for parallel index scans

Attached patch goes a bit further with simplifying _bt_first's
handling of seizing the parallel scan. This continues recent work from
commits 4e6e375b and b5ee4e52.

Aside from requiring less code, the new structure relieves _bt_first
from having separate calls to _bt_start_array_keys for the serial and
parallel cases. It also places emphasis on the idea that it's expected
that calls to _bt_first will go through _bt_readfirstpage (not
_bt_readnextpage). While it is of course possible for a parallel
scan's _bt_first to call _bt_readnextpage instead, that is the
exception, not the rule.

--
Peter Geoghegan