cross_period_persistent_keys should not be enforced to be set

[dvilelaf]

packages.valory.skills.abstract_round_abci.base.ABCIAppInternalError: internal error: Cross period persisted key mech_responses was not found in the db but was required for the next period

We currently require that cross_period_persistent_keys exist after a period finishes. There are usecases where this might not be idea. cross_period_persistent_keys should (as the name says) be keys that are not cleaned up during the reset, but shouldnt be enforced to always exist.