refactor: init indexes, decouple 'Start()' from the creation step

No behavior change.

The goal here is to group indexes, so we can perform the same
initialization and verification process equally for all of them.

The checks performed inside `StartIndexes` will be expanded
in the subsequent commits.

src/init.h

@@ -73,4 +73,7 @@ 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);
+
 #endif // BITCOIN_INIT_H