Just pre-allocate log files instead of recycling. #1373

@michaelcahill Please review this modified branch for log files.  The code is cleaner to dis-associate archiving from prepping log files for use (other than they happen in the archive code path).  This is built on the other branch for the rest of the change.

I'm only lukewarm on the `prepare` name.  
Maybe `preallocate=int`, defaulting to ~2, meaning that the archive thread will always try to keep 2 log files ready for use?