name_show and name_filter should fail by default if blockchain is incomplete

[JeremyRand]

I wrote a patch to NMControl which rejects incomplete blockchains: https://github.com/JeremyRand/nmcontrol/tree/incomplete_blockchain . However, it slows down lookups due to making 4 RPC calls per lookup instead of 1. This would be much more efficient to do on the namecoind side. Outdated name data is dangerous because it could include TLS fingerprints that were later revoked, or even the wrong owner of a name, both of which could be exploited to cause a MITM. It's also confusing to users because some websites will work fine, while others will be mysteriously broken.

I propose that name_show and name_filter return an error by default if the blockchain is incomplete (still downloading), or if errors were encountered (e.g. "you may need to upgrade"). An optional parameter (disabled by default) could enable name lookups regardless of this condition.

I'm ambivalent on whether this should be added to legacy namecoind or directly added to libcoin (assuming we're planning to use libcoin long-term).

[domob1812]

This sounds like a good idea. I think there's already such a check present (see "safe mode"), and we could easily implement it in namecoind itself. Huntercoin already has this for querying game states, AFAIK.

[phelixbtc]

+1