move circuitv2 and relay packages

[marten-seemann]

This doesn't really belong in host, that's for host implementations. "relay" is there because it used to be a host wrapper (it really should move).

Maybe move both to a "relay" directory? Really, the "relay" belongs in some form of "relay" package, and the "client" part belongs in a "transports" package, but we don't need to get too fancy.

Originally posted by @Stebalien in #1174 (comment)

[vyzo]

We should have it in protocol I think, it doesn't belong in host and we don't need a separate package for this.

[vyzo]

Note that relay contains the autorelay code, it should probably be renamed as such and maybe move into protocol as well.

[marten-seemann]

This should have been resolved since v0.16.0.