GUIDs are not great to work with. "B900A5B8-2322-11DD-A835-2B9E427B83F6" is a
lot to demand of a user. And god forbid they have to actually type in that
meaningless string. Substring matching doesn't help much because two GUIDs can
easily differ by only a bit.

Instead, we give users local IDs for each record. So instead of
"B900A5B8-2322-11DD-A835-2B9E427B83F6" they might get "7". Local IDs are local
to a replica - so two users can have different records with local ID "7".

Because local IDs are integers, they're always distinguishable from global IDs.

Local IDs are mildly fleeting. They're contained in a single file (directory?)
which may be removed at any time. Every time a record is loaded, it's given
a local ID which is cached so the user may use it.

The local ID -> global ID mapping is contained in the $replica/local-id-cache
file (directory?).

