Improve NodeStore to ShardStore imports

* Run the import process in a background thread
* Prevent online_delete from removing ledgers pending import

src/ripple/rpc/impl/Handler.cpp

@@ -107,6 +107,10 @@ Handler const handlerArray[]{
     {"log_level", byRef(&doLogLevel), Role::ADMIN, NO_CONDITION},
     {"logrotate", byRef(&doLogRotate), Role::ADMIN, NO_CONDITION},
     {"manifest", byRef(&doManifest), Role::USER, NO_CONDITION},
+    {"nodetoshard_status",
+     byRef(&doNodeToShardStatus),
+     Role::USER,
+     NO_CONDITION},
     {"noripple_check", byRef(&doNoRippleCheck), Role::USER, NO_CONDITION},
     {"owner_info", byRef(&doOwnerInfo), Role::USER, NEEDS_CURRENT_LEDGER},
     {"peers", byRef(&doPeers), Role::ADMIN, NO_CONDITION},