Add Reporting Mode

* Add a new operating mode to rippled called reporting mode
* Add ETL mechanism for a reporting node to extract data from a p2p node
* Add new gRPC methods to faciliate ETL
* Use Postgres in place of SQLite in reporting mode
* Add Cassandra as a nodestore option
* Update logic of RPC handlers when running in reporting mode
* Add ability to forward RPCs to a p2p node

src/ripple/rpc/impl/ServerHandlerImp.cpp

@@ -491,6 +491,9 @@ ServerHandlerImp::processSession(
     }
     else
     {
+        if (jr[jss::result].isMember("forwarded") &&
+            jr[jss::result]["forwarded"])
+            jr = jr[jss::result];
         jr[jss::status] = jss::success;
     }