daemon: Instantiate one lookup formula for each segment in a path

[rekmarks]

Lookup formulas currently correspond to a pet name path of length at least 2, and the value resolved thereby. We should instead create a lookup formula for each path segment, and store a reference to the parent segment formula in each child. Ideally, we should be able to synchronously retrieve existing formulas for a given pet name path, provided that the path is local to the current daemon.

[rekmarks]

@kriskowal, I believe we originally planned to derive child segment formula numbers from their parents, but #2089 removed derivation in favor of randomly generating all formula numbers. Can we reintroduce derivation for this purpose? It seems problematic if we're also trying to ensure that formula numbers are never reused.