mgr: enable inter-module calls

mgr: enable inter-module calls

This is being done by passing native CPython objects
back and forth.  It's safe because sub-interpreters in CPython
share memory allocation infrastructure and share the GIL.

With a view to PEP554, we limit inter-interpreter calls
to pickleable objects, so that this may be implemented
using byte-arrays in future.

This infrastructure should enable:
 - the dashboard to display the status of other modules, for
   example the set of progress indicators from `progress`
 - dashboard and restful to share an underlying long running
   job mechanism.

Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit f02316adb4baf4dceaab79cc0ef4c2acdb544f3e)