[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.

src/wallet/export.rs

@@ -73,7 +73,7 @@ use serde::{Deserialize, Serialize};
 
 use miniscript::{Descriptor, ScriptContext, Terminal};
 
-use crate::blockchain::Blockchain;
+use crate::blockchain::BlockchainMarker;
 use crate::database::BatchDatabase;
 use crate::wallet::Wallet;
 
@@ -115,7 +115,7 @@ impl WalletExport {
     ///
     /// If the database is empty or `include_blockheight` is false, the `blockheight` field
     /// returned will be `0`.
-    pub fn export_wallet<B: Blockchain, D: BatchDatabase>(
+    pub fn export_wallet<B: BlockchainMarker, D: BatchDatabase>(
         wallet: &Wallet<B, D>,
         label: &str,
         include_blockheight: bool,