XD-2105: Make SI property accessors available to bus partition selection

This does the job, but for the special case of json property access, we're again hit by the weirdness of `org.springframework.integration.json.JsonPropertyAccessor$ToStringFriendlyJsonNode`: One has to use *e.g.* 
```
module.http.producer.partitionKeyExpression=payload.household.toString()
```
 instead of
```
module.http.producer.partitionKeyExpression=payload.household
```
 as we get a new instance of ToStringFriendlyJsonNode every time.

At the very least, we should override `equals()` and `hashCode()` on `ToStringFriendlyJsonNode` to delegate to the wrapped node, if not get rid of that approach completely. Will open a JIRA against SI for that