Added proxy state + hibernation support

In this pull request a new state "proxy" is introduced to give an opportunity dynamically switch from synchronized mode using active/notify to "proxy" mode when msgs are send as soon as they arrive to PO Box(both sending those which exist in the buffer if any). This is done in order to reduce amount of messages between PO Box and consumer when the load is low. The other functionality remains the same. Also I added support for hibernation of PO Box process and fixed an inaccuracy in the active/3 doc.