Move the definition of JQueue objects to a separate file

Move all the definition of objects to a spearate file. In
this way, the lense module for JQueue can use these objects,
while JQueue can use the lenses. For use outside, we reexport
the objects.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>