indexes, refactor: Pass Chain interface instead of CChainState class to indexes

Passing abstract Chain interface will let indexes run in separate
processes.

This commit does not change behavior in any way.

src/node/interfaces.cpp

@@ -781,6 +781,7 @@ public:
         return Assert(m_node.chainman)->IsSnapshotActive();
     }
 
+    NodeContext* context() override { return &m_node; }
     NodeContext& m_node;
 };
 } // namespace