IPNS Key Revocation

There is currently no scheme in place for revoking IPNS Keys. Revocation is important since IPNS security relies on the security of an underlying asymmetric key. However, what happens when those keys are deemed no longer secure (e.g. RSA-1024 no longer deemed secure, discovered bug in RNG used to generate keys, laptop stolen...).

One scheme that's pretty easy to implement is reserving the sequence number MaxUInt64 as indicating that an IPNS record has been revoked. It's basically implied anyway because you cannot publish updates after sequence number MaxUInt64, however this would embed in the spec that we should return an error and not the data if the record has been revoked. There are also other potential options we could explore if we're interested.

@lidel @hugomrdias @Stebalien Thoughts?