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/interfaces/chain.h

@@ -304,6 +304,10 @@ public:
 
     //! Return true if an assumed-valid chain is in use.
     virtual bool hasAssumedValidChain() = 0;
+
+    //! Get internal node context. Useful for testing, but not
+    //! accessible across processes.
+    virtual node::NodeContext* context() { return nullptr; }
 };
 
 //! Interface to let node manage chain clients (wallets, or maybe tools for