Add WatchRegistration function

[horkhe]

The library only provided WatchInstance function that returned a watch of child type. But child type watches are triggered by group member subscription updates, so WatchRegistration function was introduced to allow watching member subscription updates.

Besides:

• There was no way to update an existing subscription so clients had to make a Deregister/Register sequence, that resulted in extra watch trigger. So UpdateRegistration function was added to rectify this deficiency.
• A redundant call of Exists was removed in a couple of methods. It seems better to just make whatever call we need and check for ErrNoNode error instead.

[horkhe]

@wvanbergen I am not sure if you still maintain this, but if so please take a look.

[wvanbergen]

Looks good, thanks for contributing!