Add query for the creation time of consumer clients

[mpoke]

Problem

Consumer chains need to set the genesis_time in their genesis file. Currently, a consumer chain could set this to the spawn_time, however this can be a timestamp in the past and it doesn't really reflect the time when the consumer client was actually created. To facilitate the launch of consumer chains, we need to introduce a query for the timestamp when a consumer client was created. See #690 for more details.

Closing criteria

The creation time of consumer clients can be queried.