Implement Netconf Mount Point Manager in sal-netconf-connector

Problem - A new MountPointContext object is created whenever any device gets connected, even same type device, this consume much memory space if connected device count is higher.

Solution - To overcome this problem, we can add functionality to maintain only one MountPointContext object if device is same (same capabilities )

Example - if 1000 Cisco ASR 1000 devices and 1000 Juniper mx960 devices get connected then only two MountPointContext should be available in memory.