Rework orphan block validation (validate_pow_only)

[antiochp]

We call validate_pow_only() early in the block processing pipeline to prevent invalid blocks being added to the orphan block pool but we can do this earlier and in a cleaner way.
We need less conditional logic around these validation steps.

This PR updates the initial validation steps in process_block so we call validate_pow_only() before hitting the db for chain head and previous header.
Also took the opportunity to clean validate_pow_only() up a little to read better.

Note: We are in the unfortunate position of having tests dependent on headers with invalid pow. We need to return early in validate_pow_only() if the ctx includes SKIP_POW. This is only used in tests.

We should make a note to follow up with some effort to get rid of SKIP_POW entirely.

[tromp]

solid improvement