Check disconnected flag on each operation

Check disconnected flag on each operation

We are performing a staggered setup during which we may encounter an
asynchronous disconnect. We have an atomic flag guarding this, which is
always flipped before disconnect enters critical region.

Check the flag at each synchronized connect stage and abort connection
if it is observed as flipped -- ensuring we cannot accidentally signal
channel activation after the channel has been close()d.

Also ditch the AtomicBoolean and instead use a simple VarHandle, saving
a few bytes of overhead.

JIRA: NETCONF-905
Change-Id: I857b1768b083c13ac5060a803a9ba6d4008c5e4b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>