Create the command structure for defer()
Simplify how the stack is maintained.

Some significant API changes to how Objectify is used outside of the
ObjectifyFilter.
Bump pom to 5.1.0-SNAPSHOT
Implement defer()
Bye bye support for master/slave datastore
Bump pom to 5.1.1-SNAPSHOT
Allow ObjectifyFilter to be reentrant

Dispatch forwarding causes the ObjectifyFilter to be run multiple
times. This means we can’t alert users if they create multiple
objectify contexts.
Another reentrancy issue with ObjectifyFilter

We can’t be sure the stack should be empty when when unwind the context.