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[David Bremner]

Here's one approach to keeping config information at the library
level.  IMHO, a key philosophical point is that this metadata is
associated with a database, not with the library.

Having every key map to a distinct file is arguably not as nice for
humans to edit, but it avoids certain concurrency complications;
e.g. glib can atomically write a keyfile (like we use for
.notmuch-config), but that means e.g. the result of two concurrent
updates to different keys is not a valid serialization.

It won't be very efficient for huge numbers of keys, but for keeping
some static metadata associated with a notmuch database, this should
work ok.

In addition to needing some polishing (documentation? what
documentation?), this isn't actually used anywhere in notmuch.