IPNS over PubSub Pinning

There is on-going work to make IPNS over PubSub have a faster initial resolution time by adding persistence to PubSub (e.g. libp2p/go-libp2p-pubsub#171). This scheme works in p2p scenarios where various users are each subscribed to the PubSub channel named after the IPNS Key. However, another use case we'd like to support involves having dedicated pinning nodes that can each provide many IPNS Keys.

**Audience:** IPNS users that want to plan to provide many IPNS keys from a single machine. This may also include IPFS pinning services interested in providing IPNS support.

**Impact:** Will enable interactions such as users publishing content to IPNS and paying someone else make sure it's available, all without giving away their private IPNS signing keys.

**Stakeholders:** go-ipfs team (and infra if we'd like to test deploying a pinner internally)