crimson/net: dispatch events in SocketConnection

crimson/net: dispatch events in SocketConnection

* move dispatch(), and exception handling logics in accept() and
  connect() from SocketMessenger into SocketConnection, so we can manage
  the state transition in the same class and at the same abstraction
  level.
* gate the dangling futures in SocketConnection, because the
  connection's smart_ptr won't be hold by messenger any more during
  exception handling.
* don't return close() inside SocketConnection to prevent recursive
  gating -- dead lock.

Signed-off-by: Yingxin <yingxin.cheng@intel.com>