Consider changing DS key generation schemas #145
Description
Having the same Datastore key prefix is considered a bad practice that leads to the Datastore performance degradation.
We now have such prefixes for LifecycleFlags in DsAggregateStorage. The other prefix-based key is used for Snapshots.
In DsProjectionStorage we also have such a prefix for last handled event time. An additional note for DsProjectionStorage is that we use google.protobuf.Timestamp as a kind for the last event timestamp which is misleading.