Implement Directory layer

[kirilltitov]

That's an oof, because I don't really understand how to port it from Python or Go, because it utilizes [blocking, obviously] mutexes heavily, and it's not quite clear

1. Why use them
2. How to use them [in Swift-NIO context]

Appendix:

1. High-level documentation: https://apple.github.io/foundationdb/developer-guide.html#directories
2. Python implementation: https://github.com/apple/foundationdb/blob/master/bindings/python/fdb/directory_impl.py
3. Go implementation: https://github.com/apple/foundationdb/blob/master/bindings/go/src/fdb/directory/allocator.go

[dimitribouniol]

1. Why use them

It seems like the primary reason they would be used is so that if you have a large subspace prefix, that can be abstracted away to a short prefix instead via indirection.

[kirilltitov]

No, the question was about mutexes, not dictionary layer :)