[Persistence-sig] A simple Observation API

[Jeremy Hylton]

> DM2.prepare()
> DM3.prepare()
> DM1.prepare()
> 
> DM2.vote()
> DM3.vote()
> DM1.vote()

Note in the API I've proposed/implemented, there is only prepare(),
not vote().  The resource manager should return True from prepare() if
it is prepared to commit.

Jeremy