server: make node join safe.

Now the node join process is:
1. verify data dir is empty.
2. verify self does not exists in member list.
3. add self to member list by calling `addMember`.
4. run server.

There is a potential risk if something goes wrong between step 3 and 4, then the new node is in member list, while data dir is still empty, so it will not be able to join. What's worse, if there is only 1 node before the join, the pd cluster will stop serving due to lack of online nodes.