WT-2823 made changes so that WiredTiger can operate with append-only filesystems (without truncate support).

On review sue.loverso noticed that log traversal depends on truncating files. Traversal will fail if a log file has been extended, partially filled, then a subsequent log file has been created. The empty space at the end of the first log file will cause a log scan to fail.

We need (a) a test case for this situation that can run with and without truncate, and (b) a solution / workaround.