Refactor restconf.common.patch

Patch utilities for RESTCONF have a number of down sides:

• @Patch is defined here, making the entire package JAX-RS-dependent
• naming should reflect the fact the binding is to NormalizedNode
• some constructs (like PatchOperation) should be taken directly from binding-generated code
• PatchEntity is really an edit
• PatchContext is tied to InstanceIdentifierContext, making it hard to pass down to contexts which are not tied to it (while retaining patch-id, etc.)