NETCONF scalability: share instances

Scalability testing shows that the netconf session code:
- retains XML document reference
- does not reuse QName instances

Change-Id: I6b58355e3fa74d67872d665d0f8dc2d16abe156c
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "NETCONF scalability: share instances"