msg/async: add timeout for connections which are not yet ready

msg/async: add timeout for connections which are not yet ready

There could be various corner cases that may cause an async
connection stuck in the connecting stage (e.g., by manually
creating some loop back connections on the switches of our test cluster,
we can almost 100% reproduce http://tracker.ceph.com/issues/37499).

In 61b9432ef9a3847eceb96f8d5a854567c49bbf61 I try to employ the
existing keep_alive mechanism to get those stuck connections out of the
trap but it does not work if the corresponding connection
is not yet ready, since we always require the underlying connection to be
**ready** in order to send out a keep_alive message.

Fix by making a more general connecting timeout strategy.
If a connecting process can not be finished within a specific interval,
then we simply cut it off and retry.

Fixes: http://tracker.ceph.com/issues/37499
Fixes: http://tracker.ceph.com/issues/38493
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>