use CREATE DATABASE STRATEGY = FILE_COPY in pg_upgrade

I noticed that the "Restoring database schemas in the new cluster" part of
pg_upgrade can take a while if you have many databases, so I experimented
with a couple different settings to see if there are any easy ways to speed
it up.  The FILE_COPY strategy for CREATE DATABASE helped quite
significantly on my laptop.  For ~3k empty databases, this step went from
~100 seconds to ~30 seconds with the attached patch.  I see commit ad43a41
made a similar change for initdb, so there might even be an argument for
back-patching this to v15 (where STRATEGY was introduced).  One thing I
still need to verify is that this doesn't harm anything when there are lots
of objects in the databases, i.e., more WAL generated during many
concurrent CREATE-DATABASE-induced checkpoints.

Thoughts?

-- 
nathan