Support merging contacts being referenced in ECK entities

[jensschuppe]

If ECK entities reference contacts (through an entity reference custom field of type Contact), the merger does not seem to find those references. ECK should implement hook_civicrm_merge() to support this.

@dontub since you've already looked into that, do you have anything to add?

Potential blocker for stable version 1.0

systopia-reference: 24496

[dontub]

@dontub since you've already looked into that, do you have anything to add?

No, currently not.