Add property to get context_id to Async

Add a property to get the context id, when set, from an Async.  If it
is not set, a NotInContextError will be raised instead.
Add simple ndb_persitence Context storage model
Update store_context signature to pass in the Context

Pass the whole context into store_context rather than just the id and
options dict.  This should be more flexible and extensible since helper
functions will be nicely exposed rather than digging into the options
dict directly.
Add simple store_context function to ndb_persistence