Avoid spurious SessionCloseError when intentionally closing session.

[glennmatthews]

Noticed that when sending a CloseSession RPC, which calls session.close(), ncclient still logs a debug about SessionCloseError("Unexpected session close") and calls close() a second time.

Solution: Add a _closing event and set its flag when close() has been called already; if thread detects that the session is closed and this flag is set, treat it as a graceful exit rather than raising an exception.

[coveralls]

Coverage increased (+0.08%) to 69.131% when pulling ace0cae on glennmatthews:spurious-session-close-error into 8ef20f2 on ncclient:master.

[coveralls]

Coverage increased (+0.03%) to 65.836% when pulling 9569a55 on glennmatthews:spurious-session-close-error into fb094c7 on ncclient:master.