We previously hacked around the import/export functionality being missing
in the statemgr layer after refactoring, but now it's been reintroduced
to fix functionality elsewhere we should use the centralized Import and
Export functions to ensure consistent behavior.
In particular, this pushes the logic for checking lineage and serial
during push down into the state manager itself, which is better because
all other details about lineage and serial are managed within the state
managers.
In practice these pairs of functions are often used together when working
with a "full" statemgr, so these helper wrappers allow us to do that more
conveniently.
This also introduces a new interface statemgr.Storage, which represents
a state manager that has all of the storage capabilities but does not
necessarily support locking. In practice callers will usually just use
statemgr.Full, but these more-specific interfaces allow us to reflect
in APIs which subset of the statemgr functionality each function depends
on.
Martin Atkins