unix: guarantee write queue cb execution order in streams

The problem is that currently the canceled write callbacks
are being called before the already written callbacks, even
though they were queued after them.
This might need more than one iteration to happen, run it in a loop until connection_cb is called. Test failure: http://jenkins.nodejs.org/job/libuv-pullrequest/lastCompletedBuild/DESTCPU=x64,label=smartos/tapTestReport/test.tap-38/