Fix a syntax error.

Signed-off-by: Manson Li <manson.li@matchstick.tv>
update doc

Signed-off-by: Manson Li <manson.li@matchstick.tv>
Fix some bugs in ReceiverSession

1. fix some syntax errors
2. fix the logical error of heartbeat
3. delay sending 'senderconnected' to ReceiverSession

Signed-off-by: Manson Li <manson.li@matchstick.tv>
Optimize 'DELETE' interface

1. add force stopping api
2. sender session could disconnect from DIAL server
3. allow sender to close a certain application instance

Signed-off-by: Manson Li <manson.li@matchstick.tv>