Handle TokuMX oplog refs

When an op is very large (e.g., an insert of a very large document), TokuMX uses a secondary collection (local.oplog.refs) to store the full contents of the op, and puts only a reference to the full contents into the oplog. This patch modifies the Slurper to handle such oplogs correctly, and also includes a test for it which confirms that inserting a ~3MB document creates an oplog entry with a ref to the full document contents, and that the document is successfully indexed.