AtxBuilder should validate initial PoST loaded from DB

[poszu]

Description

When ATX Builder starts, it first determines whether it needs to build an initial PoST proof. It doesn't need to when the smesher already has previous ATXs or the proof is found in the local database. However, in the latter case (PoST in local DB), it could be that the proof is not valid anymore for the current configuration. It could happen if the user increases Space Units.

Steps to reproduce

1. initialize X SU
2. wait for the initial PoST to be generated
3. increase to X+N SU
4. the initial PoST proof in the DB is no longer valid

Actual Behavior

Atx Builder trusts the initial PoST found in the local DB. It later repeatedly fails to publish an ATX because of invalid initial PoST proof. It cannot break out of the cycle on its own, the only way to get out is to manually remove the local.sql DB.

Expected Behavior

Atx Builder should verify the initial PoST found in the local DB and recreate it if it's not valid anymore.