Static validation for Delete Channel

When the front-end requests the deletion of a certain channel, the back-end shall execute some "static" validation if the specified channel can be deleted. This validation shall include a check to see if all HST mount nodes referring to the to-be-deleted channel are leaf nodes. If not (i.e. is any of them has child nodes), deletion of that channel channel be rejected.

There is no need to check if the channel is locked, deleting a channel will also delete all locks on that channel.