For os resolver, ctrld queries against all servers concurrently, and get
the first success result back. However, if all server failed, the result
channel is not closed, causing ctrld hang.
Fixing this by closing the result channel once getting back all response
from servers.
While at it, also shorten the backoff time when waiting for network up,
ctrld should serve as fast as possible after network is available.
Updates #34
