solution: delegate bitcoin balance request to another upstream, if data is unavailable locally

docs/04-upstream-config.adoc

@@ -125,6 +125,7 @@ Options (default or as part of upstream config):
 provider such as Infura, but disabling it is not recommended for a normal node)
 | `min-peers` | 3 | specify minimum amount of connected peers, Dshackle will not use upstream with less than specified number
 | `timeout` | 60 | timeout in seconds after which request to the upstream will be discarded (and may be retried on an another upstream)
+| `balance` | `true` for ethereum, `false` for bitcoin | specify if this node should be used to fetch balance for an address
 |===
 
 === Connection type

docs/reference-configuration.adoc

@@ -103,6 +103,9 @@ cluster:
               password: 1a68f20154fc258fe4149c199ad8f281
     - id: bitcoin
       chain: bitcoin
+      options:
+        # use the node to fetch balances
+        balance: true
       connection:
         bitcoin:
           rpc:
@@ -110,6 +113,9 @@ cluster:
             basic-auth:
               username: bitcoin
               password: e984af45bb888428207c290
+          # uses Esplora index to fetch balances and utxo for an address
+          esplora:
+            url: "http://localhost:3001"
     - id: remote
       connection:
         grpc: