Turtle race

Michael, this is the bug fix for #554.

I re-ordered the turtle file create and the metadata file create so the existence of a turtle file implies a known-good metadata file.   This means I have to generate the default metadata configuration information when the the metadata file is created (instead of reading it from the turtle file), but that doesn't seem awful, and I was able to isolate all of that work in the turtle routines instead sharing it between the turtle and metadata routines.  This also simplifies wiredtiger_open() a little.

I think this is ready to pull if you agree with the solution.