* Introduce slot->BlockRef mapping for finalized chain
The finalized chain is linear, thus we can use a seq to lookup blocks by
slot number.
Here, we introduce such a seq, even though in the future, it should
likely be backed by a database structure instead, or, more likely, a
flat era file with a flat lookup index.
This dramatically speeds up requests by slot, such as those coming from
the REST interface or GetBlocksByRange, as these are currently served by
a linear iteration from head.
* fix REST block requests to not return blocks from an earlier slot when
the given slot is empty
* fix StateId interpretation such that it doesn't treat state roots as
block roots
* don't load full block from database just to return its root
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