Previously, it was only possible to set the height indirectly by calling
create_coinbase outside the create_block function. This is fine, but
verbose and not needed.
Just like hashprev can be set directly (instead of using the value from
tmpl), allow height to be set directly.
Also, use it in one place. The other places are done in a scripted-diff.
Also, add a unit test for the new feature.
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