Webfiles should not always be (re)imported at startup

The HCM ConfigurationService always invokes the WebFilesService.importJcrWebFileBundle(...) for every webfileresourcebundle definition.

This is inefficient as the WebFilesService currently force reloads such WebFilesBundle (delete/insert) entirely. Furthermore, this will be destructive on any production runtime WebFiles changes. 

With the old esv bootstrap this wasn't a problem because this could be controlled through an initialize-item, but no longer through HCM configuration.

To fix this the WebFilesService should be optimized to calculate (and store) a digest of a bundle, so it can prevent re-importing webfilebundles which didn't change.
Note that any bundled/deployed changed webfilebundle then still will be destructive on any runtime WebFiles changes.