Write smtp-filter for postfix

Every Doichain-dApp in confirm-mode holds data of it's own recipients and allowed senders from outside. If an email comes in, postfix should check over REST-API verify call, if this record exists and if the Double-Opt-In permissions are valid.

So far Doichain-dApp has a REST-API "verify" interface which takes 4 arguments instead of only two! This call works on all dApps because it checks directly inside the blockchain. https://github.com/Doichain/dapp/blob/master/doc/en/rest-api.md#verify-opt-in

We need to add a second verify call which takes only 2 arguments (recipient_mail & sender_email) which will work only at the Doichain-dApp of the responsible (trusted third party) Node.

Please see https://bounties.doichain.org/ how to apply for Doichain bounties.