don't handle v1 protocol (at least not by default)

[rade]

In order to fix #912 we changed the protocol in #1098, bumping the version to v2. For backward compatibility, the router can still also speak v1. That means the vulnerability identified in #912 is still present, unless the router is configured with --min-protocol-version=2, since an attacker can simply claim to only speak v1. So we should either remove v1 protocol completely, or change the default value of min-protocol-version to 2. If we do this in weave 1.2, this gives users a clean rolling upgrade path, from 1.0, i.e. they can first roll out an upgrade to 1.1, which speaks both protocol versions, and then to 1.2.

[bboreham]

Some thoughts on how to do the change at #1396 (comment)