Metadata crash consistency

If there is a crash during a WT_SESSION::create, WT_SESSION::drop or WT_SESSION::rename operation, WiredTiger can end up with metadata entries that have no corresponding file.

This may not be fully fixable for rename without making these operations transactional, but for create and drop, we should be able to order operations (including filesystem, log and metadata flushes) so that metadata is left consistent regardless of crashes during the operation.