Slight refactoring of the lifecycle of data

With this patch, we change slightly the manner in which we handle
data change events (including connections)

Previously, when we got a DataChangeEvent we'd try the following

1)  connect(changes) - do all the work for connecting, including
     registeringCallbacks
2)  connectionUpdate - handle connection updates
3)  updateData - use transact to write any data changes to the ovsdb instance
4)  disconnect - disconnect anything if needed

Now we have changed this so that
a)  #1 (connect) - does *not* registerCallbacks
b)  Added a 5) registerCallbacks

The reason this was done is so that the normal pipeline for writing data
can handle flushing the externalId for the OpenVSwitch table down to ovsdb
*before* we registerCallbacks and have it come back to the operational data
store.

Change-Id: I105dd01bf87b1be170e2c4082a7914fba2002249
Signed-off-by: Ed Warnicke <hagbard@gmail.com>