This Pr implements some prerequisite changes for #34004 : split the
`CachingDB` into a `MerkleDB` and a `UBTDB`, so that very different
behaviors don't clash as much.
The transition isn't handled by this PR, but after talking to Gary we
agreed that `UBTDB` should receive another `triedb`, which will only be
loaded if the `Ended` flag is set to false in the conversion contract.
If this is too hard to achieve, it makes sense to load it regardless,
and then loading can be prevented at a later stage by adding a
`UBTTransitionFinalizationTime` in `ChainConfig`.
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Co-authored-by: Gary Rong <garyrong0905@gmail.com>
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