[blockchain] Simplify the architecture of blockchain traits

[blockchain] Simplify the architecture of blockchain traits

Instead of having two traits, `Blockchain` and `OnlineBlockchain` that need
to be implemented by the user, only the relevant one (`OnlineBlockchain`, here
renamed to `Blockchain`) will need to be implemented, since we provide a
blanket implementation for the "marker" trait (previously `Blockchain`, here
renamed to `BlockchainMarker`).

Users of the library will probably never need to implement `BlockchainMarker`
by itself, since we expose the `OfflineBlockchain` type that already does
that and should be good for any "offline" wallet. Still, it's exposed since
they might need to import it to define types with generics.