Allow SchemaContext shed YANG sources

NETCONF/L2FIB test shows that a SchemaContext instance can take up as much as 1.8MB of memory, with a lot of that overhead coming from ModuleImpl retaining a reference to the YANG file source.

Analyze usage scenarios and we can make the source lazily-loaded, such that we do not keep it in-memory. A typical case when we can load the source is when the model comes from an artifact deployed in the controller, since we can read it from the jar as a resource.