get a binary snapshot of state usually invoked by the framework
unwrap the raw value of state
recover state to a previous checkpoint usually invoked by the framework
set next checkpoint time should be invoked by the framework
update state on a new message this is invoked by user
PersistentState is part of the transaction API
Users could get transaction support from the framework by conforming to PersistentState APIs and extending PersistentTask to manage the state