By introducing the "shared rocksdb instance" concept to the backend, we
can remove the "piggybacking" mode , thus reducing the complexity of
database initialisation and opening the possibility of extending how
write batching works across kvt/aristo.
The change makes explicit the hidden shared state that was previously
hiding in closures and provides the first step towards simplifying the
"commit/persist" interface of coredb, preparing it for optimizations to
reduce the "layering tax" that `forked-layers` introduced.
* renamed nimbus folder to execution_chain
* Renamed "nimbus" references to "execution_chain"
* fixed wrongly changed http reference
* delete snap types file given that it was deleted before this PR merge
* missing 'execution_chain' replacement
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Co-authored-by: pmmiranda <pedro.miranda@nimbus.team>
