persistence engine selection encoded in id

[johnlockwood-wf]

I had the idea we could encode the persistence engine
selection in the id of the Context.

Want it to store in ndb? postfix it with a different
delimiter such as |.
5949025a659a4829947de944c8cc328f|ndb

store with memcache?
5949025a659a4829947de944c8cc328f|mem

with redis?
5949025a659a4829947de944c8cc328f|rds

children would be:
5949025a659a4829947de944c8cc328f|ndb,Bx0
5949025a659a4829947de944c8cc328f|ndb,Fn1

This scheme would allow a context to define it's persistence
engine different from the system wide setting and allow
processor and other components to know how to load
markers from the persistence layer.