rpcdaemon: move transaction/cursor hierarchy to db

[canepat]

This PR moves almost all the code present in rpc module for accessing the data stores (i.e. database and snapshots) into the db module, specifically:

• the class hierarchies rooted at rpc::ethdb::Transaction and rpc::ethdb::Cursor, capturing the abstractions to interact with local/remote transactions and cursors
• the rpc/storage package, containing similar local/remote abstractions for block/header/body/transaction access layer
• the rpc::state::LocalState and rpc::state::RemoteState, again local/remote modelling for silkworm::State implementations

This refactoring is driven by the following logic:

• move all the code related to data store and data store access into the db module, because the KV interface defines remote access to the data stores (i.e. database and snapshots), so it's exposed from db
• do this as a preliminary step before introducing support for "temporal" API subset of the KV

This work is not finished yet (some additional PR will follow) and should be considered as a starting point to further shaping the db module into a more cohesive, less coupled set of abstractions for data store access.

Extras

• move clock_time facilities to infra
• move remote/kv folder to kv folder
• move composite_storage_key to db/util
• remove many redundant namespace qualification