Refactor third-party caveat encoder/decoder to caveatid package.

Remove non-public-key third-party caveatid encoding for now (this can always be added
back later as an alternative implementation of bakery.CaveatId{Encoder,Decoder}).
Updated httpbakery & tests accordingly.
or

     p.Locator = PublicKeyLocatorMap(nil)

to make it clear that it will never have any entries in (and save a tiny bit of memory).