Use one transaction while reading postgres.bki, not one per line.

AFAICT, the only actual benefit of closing a bootstrap transaction
is to reclaim transient memory.  We can do that a lot more cheaply
by just doing a MemoryContextReset on a suitable context.  This
gets the runtime of the "bootstrap" phase of initdb down to the
point where, at least by eyeball, it's quite negligible compared
to the rest of the phases.  Per discussion with Andres Freund.

Discussion: https://postgr.es/m/9244.1492106743@sss.pgh.pa.us