This submission contains the fixes for the following 3 problems:
1. [ONOS-5675] RESTCONF clients may stop receiving notifications when multiple client
sessions are opened simultaneously
2. [ONOS-5676] RESTCONF client hangs on event notification subscription
3. bug fix for empty-leaf node
Change-Id: Icf6848d18e0599842d4889d5aa5443c58f23aeeb
Adding code to support RESTCONF protocol as one of
the supported SBI protocols of ONOS. This RESTCONF SBI extends
the current REST SBI protocl and adds some new APIs/functinalities
so that a provider can subscribe/register to an external restconf
server to receive notification stream.
Change-Id: I21bf0d0f0394cf788e066d743b3ade04735fe07e