Validate ATX eagerly when building/publishing it

[fasmat]

Description

At the moment ATXs are published immediately after building them leaving the task of validation to go-libp2p validation routines. This has a few disadvantages:

• if ATX fails validation libp2p doesn't log why validation failed
• the ATX is cached by libp2p when publishing. We had issues in the past where the ATX failed publication once, the node recreated PoST for that ATX and published it again. libp2p then reported back that the publication was successful (since the ATX was already cached) masking the problem and complicating finding the cause of the issue
• any ATX not published by an identity managed by the node must be stored when it passed syntactical validation, even if it fails contextual validation, where the node instead should publish a malfeasance proof for the malicious ATX. In contrast to that ATXs published by an identity managed by the node should only be stored when they pass both syntactical and contextual validation and malfeasance proofs should never be published against one self.

Affected code

Code in the activation package. Specifically the Builder and Handler code.