feat(cu): only allow checkpoint with hashchain, assignment id

[jfrain99]

Background

Since hashchain is a recent implementation, many older Arweave checkpoints don't contain the hash chain or the assignment id. When they are absent, the CU simply skips over the hash chain validation. This can lead to sync errors in the CU and messages being evaluated out of order.

Solution

Add an opt-in config variable that disallows using checkpoints missing these values. If a checkpoint missing them is found, skip over it and continue searching.