Block sync hash traversal perf

[antiochp]

Resolves #3554.

When we transition from "header sync" to "body sync" we identify missing blocks based on the delta between the chain of headers and the current chain of full blocks. i.e. We sync headers and then we go request the full blocks for each of those headers.
There is some additional complexity where we support a fork between the header chain and the full block chain as one is not always a subset of the other. And to do this we identify the "fork point" which is the last common header between header chain and full block chain before they begin to diverge.

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Fork example -

Headers: A -> B -> C' -> D -> E

Full blocks: A -> B -> C

Header sync gave us a chain of headers building on C' while our local node was previously aware of C.
We need to "rewind" to the fork point B immediately prior to C and C' and request missing blocks [C', D, E].

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The existing implementation rewinds back from the head of the header chain to identify both the fork point and a set of missing block hashes. This is fine if the delta between header chain and full block chain is small but is not suitable if the header chain must be rewound a significant number of blocks.

It fails badly in the "full archval node sync" scenario where we must rewind from height ~1,500,00 back to 1.

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The PR introduces a more efficient approach -

1. identify height of full block chain
2. increase this height by the number of blocks we will request in parallel
3. lookup this "max" header directly on header chain via get_header_by_height()
4. Iterate back from this "max" header to the fork point.

This places an upper bound on the iteration required as we can lookup headers directly based on height.
This greatly reduces the amount of iteration required when identifying missing blocks to sync.

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As part of this rework check_txhashset_needed() was refactored, splitting out get_fork_point() and check_txhashset_needed() into separate fns.
This allowed the implementation to be simplified significantly.

[antiochp]

Going to merge this to master.
We need to make sure we test sync thoroughly as part of the next release.