RPC: Rename 'Restore' to 'Bootstrap' and redefine it to have nothing to do with SturdyRefs.

The 'objectId' field is now deprecated. Long-term, each vat will export no more than one "bootstrap interface" which can be obtained via 'Bootstrap'. Restoring SturdyRefs will be accomplished through higher-level interfaces specific to the VatNetwork in use.

See comments for 'Bootstrap' in rpc.capnp for more discussion.
Merge branch 'master' of github.com:kentonv/capnproto