organize policy tags into classes

This would allow, for example, disabling bitcoind related transaction relay checks, if you use some other mechanism for transaction inclusion.

Further motivated by @ariard's message:

The policy rules can be bypassed one by one with 'PolicyFilter' if they match the appropriate tag, however I don't think there is a notion of "class" to disable let's say all the transaction-relay policy checks, if you're targeting another full-node (e.g btcd) as a broadcast channel. I think it could be also valuable to introduce the class of consensus validation checks and just redo the stateless one in Bitcoin Core with 'CheckTransaction' (src/consensus/tx_check.cpp). Core's bitcoinconsensus only proposes script verification