cli: deprecate neighbor `shutdown` command
cli: deprecate neighbor `shutdown` command

Since `shutdown` command just sends ADMINISTRATIVE_SHUTDOWN notification
and doesn't change admin-state, neighbors can be get established after
idle-holdtime which is not intuitive from the name of the API.

We can use `reset` command to hard-reset BGP session, and `disable` command
to shutdown(disable) BGP session.

Signed-off-by: Wataru Ishida <15703620d0b11e835ab4f34ebf88f2f40cce783e@lab.ntt.co.jp>

fujita/gobgp,a16/gobgp,j-kato/gobgp,Dipsingh/gobgp,fujita/gobgp,osrg/gobgp,a16/gobgp,Dipsingh/gobgp,osrg/gobgp,a16/gobgp,j-kato/gobgp,a16/gobgp,a16/gobgp,tamihiro/gobgp,osrg/gobgp,a16/gobgp,a16/gobgp,fujita/gobgp,fujita/gobgp,ishidawataru/gobgp,osrg/gobgp,j-kato/gobgp,tamihiro/gobgp,tamihiro/gobgp,ishidawataru/gobgp,ishidawataru/gobgp,Dipsingh/gobgp,tamihiro/gobgp