Ensure existing yang schemas are pre-loaded on startup

This bug came from https://bugs.opendaylight.org/show_bug.cgi?id=1815.

The ModuleInfoBackedBundleTracker listens for ACTIVE bundles and extracts the YangModuleInfo(s) from each bundle. This is used to subsequently extract the yang files to build the SchemaContext.

The problem is that on startup, a client may get an incomplete SchemaContext if some yang module bundle(s) haven't activated yet. This can lead to failures and affected clients have to work around (e.g. distributed data store).

I think it would be ideal if we could ensure all existing schemas are loaded on startup prior to clients accessing the SchemaContext. This can be achieved if ModuleInfoBackedBundleTracker listened for RESOLVED bundles as the OSGi container first resolves all bundles before activating them. I've used this approach previously.