Support publishing self-records in LAN DHT even when WAN DHT is available.

As I mentioned in #7109, I am part of @tomeshnet, and we use IPFS on our experimental mesh network nodes. There are some new changes in the upcoming v0.5.0 release that will affect us, that I discussed with @Stebalien and @willscott in that issue, eventually leading me to create this feature request.

> The only question I have now is whether there's a way to disable the setting that means nodes "will only publish records (provider and IPNS) to the WAN DHT". In a situation where a node is connected to the Internet and CJDNS, for example, we'd want them to still be publishing records to the LAN (CJDNS).
> Or more specifically, we'd want them to publish any of their own records to the LAN, but not WAN records that a CJDNS-only node couldn't access.
> -- [me](https://github.com/ipfs/go-ipfs/issues/7109#issuecomment-614200399)

@Stebalien informed me this is not possible, and so I'm asking if this can happen, through a config option, as well dynamically through an `ipfs` command.

To be clear, when this option is enabled on a node, then anytime a node is publishing records on the WAN DHT for blocks/IPNS addresses that it has, it should also publish those records on the LAN DHT. It should not publish records that point to WAN nodes.

The endgoal of this is that if you have node **A** that is connected to the Internet and a private mesh network like CJDNS, and **A** adds a file to IPFS, then **B** which is only connected through CJDNS, should be able to retrieve the file, because **A** published the record for the file on the LAN DHT that **B** was a part of.