Improve validation when processing orphan blocks

[j01tz]

The block acceptance pipeline can potentially be improved by adding a proof of work validation check before adding orphan blocks to the orphan block pool.

Currently adding an orphan block with an invalid proof of work is allowed and can consume unnecessary resources and potentially flush legitimate orphans from the pool if MAX_ORPHAN_SIZE is reached.

This PR adds a check based on the one proposed by @quentinlesceller to better validate orphan blocks before they are added to the orphan block pool.

• Adds validate_pow_only() function to cheaply validate orphans
• Adds validate_pow_only() check to pipe::process_block() inside the orphan conditional before ​Err​(ErrorKind::​Orphan​.into()) is returned

This is a WIP as it needs more testing, someone may find a more efficient/elegant way for this check and tests still need to be added.

[antiochp]

Nice. This looks good.

Not related to this PR but we may want to refactor the existing block/header validation to do the same thing earlier in the pipeline, before we start looking for previous header in the db etc.

[antiochp]

Going to do some local testing of this - will merge after that.