Create NS instance at startup time and open/close it at BTS start/stop

The NS instance must be created at startup time before the config is
parsed, so the NS VTY elements will exist at this point. The NS instance
must persist.

Then during BTS start, the NS instance is opened/bound/connected to the
socket that is specified by BTS.

Then during BTS stop, the NS instance is closed, but the NS instance sill
exists.

At PCU stop, the NS instance is completely destroyed.
3 files changed