Avoid spamming of keychain & keychain access transaction

We enhanced the validation of keychain & keychain access transactions in #882

But a more advanced security level could be applied to prevent spam. One idea can be to allow only one software origin key for keychain access per keychain, the remaining should be hardware-based thanks to the POW.
For this feature, we would need to authorize the node shared secrets (storage nonce) to be able to read the keychain address.

Start by reviewing the keychain and keychain access transaction validation introduced in #882. Trace how node shared secrets and the storage nonce can read the keychain address, then clarify the proposed software-origin and hardware-based proof-of-work rules. Done means the anti-spam policy is specified and covered by validation tests.

Written by the indexing model from the issue text.