native: never set Neo's newEpoch* cache values to nil

We have cache update mechanism (Neo's cache votesChanged flag), it must
be used for current epoch and new epoch cached values update. And the
cached current/new epoch values themselves must always contain valid
information for the current/new epoch. These cached values must only be
changed once per epoch, never set them to nil.

This commit prevents CN node panic described in #3253 when dBFT tries
to retrieve new epoch validators with some votes modifications made
before at the same dBFT epoch.

Close #3253.

Signed-off-by: Anna Shaleva <[email protected]>

pkg/core/native/native_neo.go

@@ -971,7 +971,6 @@ func (n *NEO) ModifyAccountVotes(acc *state.NEOBalance, d *dao.Simple, value *bi
 				return nil
 			}
 		}
-		cache.newEpochNextValidators = nil
 		return putConvertibleToDAO(n.ID, d, key, cd)
 	}
 	return nil