Hot backup

Michael, this branch adds hot-backup support to test/format; it's not yet ready to pull, though.

However, opening two databases in a single application (test/format opens a second database in order to verify the hot backup, in RUNDIR/BACKUP), triggered a bug in all of our compressors, where we had a static WT_EXTENSION_API reference.

If you open a database, open a second database, then close the second database, the close zero's out the information referenced by the static handle, and then the first database connection fails the next time it uses the extension api handle.