Possibly consolidate SingletonState(s) to a single KVStore pair

### Problem

We have a number of single values that must be stored in state (e.g. a single hash). These are often stored in SingletonState via an entire ReadableStore/WritableStore combination.
This appears to be excessive and creates added code and maintenance that is not strictly necessary.

### Solution

There may be value in consolidating these singular values into a single Store implementation that implements multiple interfaces defined by the various services (much as SingleTransactionRecordBuilderImpl does) to reduce the repeated code and simplify the maintenance of singleton values.

Example
```java
public class SingletonReadableStore implements StakingRewardsStore, UpgradeHashStore, RunningLeafHashStore {
@Override
@NonNull
public Bytes updateFileHash() {...}

@Override
@NonNull
public Hash getRunningHash() {...}

@Override
@NonNull
public Hash getNMinusThreeRunningHash() {...}

@Override
@NonNull
public NetworkStakingRewards getStakingRewards() {...}

@Override
@NonNull
public boolean isStakingRewardsActivated() {...}

@Override
@NonNull
public long pendingStakingRewards() {...}

@Override
@NonNull
public long totalStakeRewardStart() {...}

@Override
@NonNull
public long totalStakedStart() {...}
}
```

### Alternatives

continue with the current practice of creating a new ReadbleStore and WritableStore pair for each individual singleton value, possibly creating a new VirtualMap for each individual value as well.