sd-journal: properly handle inotify queue overflow

This adds proper handling of IN_Q_OVERFLOW: when the inotify queue runs
over we'll reiterate all directories we are looking at. At the same time
we'll mark all files and directories we encounter that way with a
generation counter we first increased. All files and directories not
marked like this are then unloaded.

With this logic we do the best when the inotify queue overflows: we
synchronize our in-memory state again with what's on disk.

This contains some refactoring of the directory logic, to share more
code between uuid directories and "root" directories and generally make
things a bit more readable by splitting things up into smaller bits.

See: #7998 #8032