Currently when load(oid) finds that the object was deleted, it raises
ReadConflictError - not POSKeyError - because a pack could be running
simultaneously and the deletion could result from the pack. In that case
we want corresponding transaction to be retried - not failed - via
raising ConflictError subclass for backward-compatibility reason.
However from semantic point of view, it is more logically correct to
raise POSKeyError, when an object is found to be deleted or
not-yet-created, and raise ReadConflictError only if a pack was actually
running simultaneously, and the deletion could result from that pack.

-> Fix MVCCAdapter.load to do this - now it raises ReadConflictError
only if MVCCAdapterInstance view appears before storage packtime, which
indicates that there could indeed be conflict in between read access and
pack removing the object.

To detect if pack was running and beyond MVCCAdapterInstance view, we
need to teach storage drivers to provide way to known what was the last
pack time/transaction. Add optional IStorageLastPack interface with
.lastPack() method to do so. If a storage does not implement lastPack,
we take conservative approach and raise ReadConflictError
unconditionally as before.

Add/adapt corresponding tests.

Teach FileStorage, MappingStorage and DemoStorage to implement the new
interface.

NOTE: storages that implement IMVCCStorage natively already raise
POSKeyError - not ReadConflictError - when load(oid) finds deleted
object. This is so because IMVCCStorages natively provide isolation, via
e.g. RDBMS in case of RelStorage. The isolation property provided by
RDBMS guarantees that connection view of the database is not affected by
other changes - e.g. pack - until connection's transaction is complete.

/cc @jimfulton