Issue #822 - Allow channel._handle_close to bypass session rekey block

[tsellers-r7]

This PR attempts to address Issue #822 opened by @mathrick

The change adds an argument to transport._send_user_message that allows the message to be sent during key negotiation. channel._handle_close is modified to set this argument to True to enable the closure of a channel even after a rekey has been requested by the client. Normally this closure message (cMSG_CHANNEL_CLOSE) would be blocked in transport._send_user_message until after the rekey process has completed. In some cases, as demonstrated in Issue #822, this never occurs and the connection errors out.

After implementing this change the demo code in Issue #822 was able to run for at least 700 loop iterations. Prior to the change the best I was able to achieve was 4 iterations before hitting the issue.

[coveralls]

Coverage increased (+0.03%) to 74.241% when pulling bb29a8c on tsellers-r7:close_channel_vs_rekey into 7366f96 on paramiko:master.

[coveralls]

Coverage increased (+0.05%) to 74.254% when pulling bb29a8c on tsellers-r7:close_channel_vs_rekey into 7366f96 on paramiko:master.

[bitprophet]

Added the parent issue to a release milestone now that there's a patch. Looks good at a glance but I need to do my usual local repro / grok step sometime. Thanks!