multi: Plugins Setup fixes

[victorgcramos]

Closes #2797

• Rename initializerIds to pluginInitializerIds
• Break the app setup if pluginInitializerIds aren't unique or if some id does not exist.
• Plugins initializers executed in parallel
• Add plugins routes integration example using /statistics route from statistics plugin.
• Move mergeRoutes util to core router
• Add docs to inform yarn is required to execute the plugin for the first time.