[wallet] Add an option to change the assumed current height

src/blockchain/mod.rs

@@ -18,6 +18,8 @@ pub enum Capability {
 }
 
 pub trait Blockchain {
+    fn is_online(&self) -> bool;
+
     fn offline() -> Self;
 }
 
@@ -26,6 +28,10 @@ impl Blockchain for OfflineBlockchain {
     fn offline() -> Self {
         OfflineBlockchain
     }
+
+    fn is_online(&self) -> bool {
+        false
+    }
 }
 
 pub trait OnlineBlockchain: Blockchain {