properly initialize event to 0 (in case no timeout is specified)
merged @yanjian timeout cleanup refactor
set discard_body=0 in timeout handler to prevent nginx worker process segfault (at least with nginx 0.8.6x)
ngx_http_push_broadcast_status_locked would never return NGX_OK, so checking the return value against NGX_OK is meaningless, instead always return NGX_OK in case of CONCURRENT_LASTIN
added a new option push_channel_timeout which controls how long a channel should be kept after accepting the last subscriber in the channel with no queued messages and no active subscriber.

this option should be used along with push_subscriber_timeout, and push_channel_timeout should be greater than push_subscriber_timeout

push_channel_timeout should appear along with push_publisher (or parent context); if push_authorized_channels_only is off, push_channel_timeout should also be set in push_subscriber context (or parent context)
fixed a small typo
Merge branch 'master' into topic/nginx-push-module/CLOUD-210