Add support for the new metadata version functionality to the directory layer

The new [metadata version](https://github.com/apple/foundationdb/pull/1213) functionality introduced in 6.1 could be helpful in the directory layer for determining whether a cached open directory has been deleted without needing to perform extra reads in every transaction. It should be possible for clients to do this themselves (i.e. update the metadata version when deleting directories and checking it when using a cached directory). However, it would be convenient if this could be provided by the layer itself with little or no client effort.

It may not be desirable to use the metadata version unconditionally, since some users of the directory layer may not need that functionality and instead want to use the metadata version for other purposes. It's also possible that a client may not want to use this functionality for all directories. For example, this may be the case if some directories are much less stable than others and aren't cached like the stable directories. We'll need to determine what level of control we want to provide and then decide on a reasonable API for it.