We were creating the turtle file, and if it didn't previously exist we'd create
the metadata file.  If we crashed between those two steps, we'd fail the next
open because the turtle file existed but the metadata file didn't.

Restructure so we don't create the turtle file until we finish creating the
metadata file (and loading any hot backup), that way if the turtle file exists,
everything else is known to be good.

Ref #554.
Merge branch 'develop' into turtle-race
Merge pull request #555 from wiredtiger/turtle-race

Make the existence of a turtle file imply a known-good metadata file.