Create more efficient and centralized subscription mechanism

Currently the subscription is from every binding, and every binding monitors the whole binding and definition sections. Improvements:
1) monitor by a central entity, which will reload corresponding binding as needed
2) monitor a designated item, instead of the whole bindings and definitions trees. For example, monitor ESDL/Subscription/Change, which is just a single value.

There were some discussions about #2 at offsite meeting. Basically we can improve the performance by monitoring a simple Dali value, or by creating another mechanism without using Dali. The consensus is to monitor a simple Dali value for simplicity, but put it in a well defined interface for possible future extensions.