refactor: index, decouple 'Init' from 'Start'

So indexes can be initialized without spawning
the sync thread.

This makes asynchronous indexes startup
possible in the following commits.

src/init.h

@@ -74,6 +74,6 @@ bool AppInitMain(node::NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip
 void SetupServerArgs(ArgsManager& argsman);
 
 /** Validates requirements to run the indexes and spawns each index initial sync thread */
-bool StartIndexes(node::NodeContext& node);
+bool StartIndexBackgroundSync(node::NodeContext& node);
 
 #endif // BITCOIN_INIT_H