ReadSystemEssentialAndBetterEarliestTime retrieves in one go
information about the model and essential snaps of the given types
for the Core 20 recovery system seed specified by seedDir and label
(which cannot be empty).
It can operate even if current system time is unreliable by taking
a earliestTime lower bound for current time.
It returns as well an improved lower bound by considering appropriate
assertions in the seed.
* asserts: Batch.CommitToAndObserve
have a variant of CommitTo that supports a callback to consider each
assertion immediately after it has been added to the database, at
which point it also verified
