Netconf stack by default locks the data store before issuing

Netconf stack by default locks the data store before issuing
edit-config, and then unlocks it.

This poses some scalabilty issues when multiple requests for
the same device arrives. Also some devices automatically
queues the requests at the device itself.

The patch provides an option to allow lock/unlock operations
for edit-config. Default value is true which defaults to today's
behavior.

JIRA: NETCONF-629
Change-Id: I0531b27c0807dfb586f9e80c9b9fc9b189e3cadf
Signed-off-by: Jakub Toth <jtoth@luminanetworks.com>
Signed-off-by: Miroslav Macko <mmacko@luminanetworks.com>