Remove an unused PersistableEnvelope interface from the following five
PersistableNetworkPayload implementations:

  AccountAgeWitness, BlindVotePayload, ProposalPayload,
  SignedWitness, TradeStatistics2

These already have corresponding *Store envelope classes which correctly
implement the interface.

Remove unused functionality to pass a custom 'toProto' function via a
constructor or setter, as it was unused and all subclasses override the
toProtoMessage() method anyway. In this way, the method may be removed
and the class (+ UserThreadMappedPersistableList) made abstract.

Also it appears PersistableHashMap was never used, so remove it.

Remove an unnecessary PersistableEnvelope interface by making them
standalone @value classes with private List fields, instead of extending
PersistableList. As they weren't using any functionality of the latter
other than the getList() and stream() methods, this should not alter
behaviour, outside MeritList::toString.

Also comment out the MERIT_LIST PersistableEnvelope protobuf message
type, which shouldn't be encountered as merit lists were never persisted
directly to a storage file.

This removes the last superfluous PersistableEnvelope implementations,
leaving the following type hierarchy:

  PersistableEnvelope *
  +- NavigationPath
  +- PeerList
  +- PersistableList *
  +- ThreadedPersistableEnvelope *
  |  +- AccountAgeWitnessStore
  |  +- BlindVoteStore
  |  +- DaoStateStore
  |  +- PersistableNetworkPayloadList              *  is abstract
  |  +- ProposalStore
  |  +- SequenceNumberMap
  |  +- SignedWitnessStore
  |  +- TempProposalStore
  |  \- TradeStatistics2Store
  \- UserThreadMappedPersistableEnvelope *
     +- AddressEntryList
     +- DisputeList *
     |  +- ArbitrationDisputeList
     |  +- MediationDisputeList
     |  \- RefundDisputeList
     +- UserThreadMappedPersistableList *
     |  +- BallotList
     |  +- MyBlindVoteList
     |  +- MyProofOfBurnList
     |  +- MyProposalList
     |  +- MyReputationList
     |  +- MyVoteList
     |  +- PaymentAccountList
     |  \- UnconfirmedBsqChangeOutputList
     +- PreferencesPayload
     +- TradableList
     \- UserPayload