[New Feature] adding to always resolve hostname with always_host_resolve option.

if set always_host_resolve to true:

It always tries to resolve when reconnect to server.

before patch
1. read conf
2. resolve host address
3. connect
4. disconnect
5. connect with 2's info

after patch("always_host_resolve: true)
1. read conf
2. resolve host address
3. connect
4. disconnect
5. resolve host address again.
6. connect with 5's info