Get block hash by its height

Create blockchain::GetBlockHash trait
with a method to get block hash given
a block height. Then, implement this
trait for all backends (Electrum, RPC
, Esplora, CBF). Referenced in issue 603.

src/blockchain/electrum.rs

@@ -98,6 +98,13 @@ impl GetTx for ElectrumBlockchain {
     }
 }
 
+impl GetBlockHash for ElectrumBlockchain {
+    fn get_block_hash(&self, height: u64) -> Result<BlockHash, Error> {
+        let block_header = self.client.block_header(height as usize)?;
+        Ok(block_header.block_hash())
+    }
+}
+
 impl WalletSync for ElectrumBlockchain {
     fn wallet_setup<D: BatchDatabase>(
         &self,