sd-journal: introduce cleanup function and hash ops for Directory

sd-journal: introduce cleanup function and hash ops for Directory

This makes the folloing:
- Each Directory object now has a reference to sd-journal.
- Hence, directory_free(), which is renamed from remove_directory(), can
  be called without sd-journal as an argument.
- Introduces hash ops for Directory, so the finalization becomes
  slightly simpler.
- Allocate hashmaps that store Directory objects when necessary.
- Split out add_directory_impl().

No functional changes, just refactoring.