Rename {Any}RelayCell to {Any}RelayMsgOuter

We're doing this renaming because we need a name for the combination of a RelayMsg and an Option<StreamId> that we use when we have a RelayMsg we intend to route to a given stream or circuit internally. Previously we called this a RelayCell, but that name was already somewhat inaccurate, and will become very inaccurate with the arrival of prop340, which breaksthe 1:1 relationship between relay cells and relay messages.

(If we didn't do this renaming now, we'd soon be making the relationship between UnparsedRelayCelland RelayCell many-to-many, which would be ridiculous and confusing.)

The RelayMsgOuter name is a placeholder: We expect that we'll want to rename this type, and may also want to rename RelayMsg, and unify our vocabulary in other areas too. But such a renaming will have to wait for a larger discussion affecting the specifications, so that we can use the same vocabulary everywhere.

Closes #775 (closed).