Handle multiple connections between two nodes

[hyunsikjeong]

Fixing #724.

Two problems:

1. A timer is not working even successfully set by io.register_timer_once(stream, random_interval).expect("Pull connections must be registered");
2. Some processes do not die normally, but main TRACE shutdown [IoService] Closed. in every log.

[hyunsikjeong]

Fixed. But I have no idea that why io.register_timer_once() is working well in finally().

[hyunsikjeong]

Two states are added in the routing table: Establishing and WaitingReconnection.
If a node get a sync message from the opposite node while Establishing, it means there are multiple connections so the node will disconnect, change the state to WaitingReconnection, and wait reconnection for random duration between 2s~5s. (See MINIMUM_RECONNECTION_TIME and MAXIMUM_RECONNECTION_TIME)

[hyunsikjeong]

Found an exception.

[hyunsikjeong]

The new issues are added (#744, #745) and the discovery test still has problems, so not allowing the test in this patch. But the patch will resolve the issue #724.
@sgkim126