parser.Stack add stored_context

Adds a new stored_context method, which may allow for some cleanup
in service.py, and also enable switching from the request context
to the stored context when needed, e.g when the current context
auth_token is about to expire.

Change-Id: I7b797719036238c424905a5d2555dc00e40f4e89
Partial-Bug: #1306294
Merge "parser.Stack add stored_context"