Fixed deadlock between AsyncSshHandlerReader and AsyncSshHandler

First was fix in AsyncSshHandlerReader but there was another
deadlock after that. Therefore a change in AsyncSshHandler is also
needed. Both ABBA deadlockw occur when AsyncSshHandler.disconnect
(..) and AsyncSshHandlerReader.invokeDisconnect(..) methods are
executed concurrently within the same SSH session that is going
to be closed.

Note: Snippets from stack-traces are from Oxygen distribution, so
the line numbers don't have to match exactly.

Deadlock #1:
"nioEventLoopGroupCloseable-3-20":
	at org.opendaylight.netconf.nettyutil.handler.ssh.client
.AsyncSshHandlerReader.close(AsyncSshHandlerReader.java:96)
	- waiting to lock <0x000000072ecffff8> (a org.opendaylight
.netconf.nettyutil.handler.ssh.client.AsyncSshHandlerReader)
	at org.opendaylight.netconf.nettyutil.handler.ssh.client
.AsyncSshHandler.disconnect(AsyncSshHandler.java:235)
	- locked <0x000000072ed00020> (a org.opendaylight.netconf
.nettyutil.handler.ssh.client.AsyncSshHandler)
	at org.opendaylight.netconf.nettyutil.handler.ssh.client
.AsyncSshHandler.close(AsyncSshHandler.java:215)
...

"sshd-SshClient[57f78db1]-nio2-thread-1":
	at org.opendaylight.netconf.nettyutil.handler.ssh.client
.AsyncSshHandler.disconnect(AsyncSshHandler.java:221)
	- waiting to lock <0x000000072ed00020> (a org.opendaylight
.netconf.nettyutil.handler.ssh.client.AsyncSshHandler)
...
org.opendaylight.netconf.nettyutil.handler.ssh.client
.AsyncSshHandlerReader.operationComplete(AsyncSshHandlerReader
.java:64)
	- locked <0x000000072ecffff8> (a org.opendaylight.netconf
.nettyutil.handler.ssh.client.AsyncSshHandlerReader)
...

Deadlock #2:
"nioEventLoopGroupCloseable-3-19":
	at org.apache.sshd.client.session.ClientSessionImpl
.signalAuthFailure(ClientSessionImpl.java:138)
	- waiting to lock <0x000000072c800218> (a java.lang.Object)
	at org.apache.sshd.client.session.ClientSessionImpl
.preClose(ClientSessionImpl.java:126)
	at org.apache.sshd.common.util.closeable.AbstractCloseable
.close(AbstractCloseable.java:95)
        at org.opendaylight.netconf.nettyutil.handler.ssh.client
.AsyncSshHandler.disconnect(AsyncSshHandler.java:251)
	- locked <0x000000072c800480> (a org.opendaylight.netconf
.nettyutil.handler.ssh.client.AsyncSshHandler)
...

"sshd-SshClient[411a406d]-nio2-thread-8":
	at org.opendaylight.netconf.nettyutil.handler.ssh.client
.AsyncSshHandler.disconnect(AsyncSshHandler.java:221)
	- waiting to lock <0x000000072c800480> (a org.opendaylight
.netconf.nettyutil.handler.ssh.client.AsyncSshHandler)
...
org.apache.sshd.common.session.helpers.AbstractSession
.handleMessage(AbstractSession.java:499)
	- locked <0x000000072c800218> (a java.lang.Object)
...

Change-Id: I42260213cad82250832451539d6ce11035ba37e2
Signed-off-by: Martin Sunal <msunal@frinx.io>
Signed-off-by: Jaroslav Tóth <jtoth@frinx.io>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>