1. In GroupFragment: when adding server replica, remove the name field,
leaving only the ip field
2. For test: In GroupFragment, click the "remove server replica" button,
you actually "are" adding two server replicas for test.
3. when the user log in successfully, start the service to listen to
connections
4. Fix bug: in the method attemptLogin() in LoginActivity, call finish()
instead of Intent to wake up the MobileMemoActivity instead of creating
a new one

Next, known bug to fix: In the method put() of KVStoreInMemory, a
NullPointerException is thrown.