Fix snapshot initialization stuck issue

[kevinkassimo]

This is a strange bug we noticed. It seems from the example of raftexample, the code has 2 bugs:

1. They replay records before loading snapshot, which would cause loss of records (snapshot overwrites store)
2. When snapshot exists, the initial blocking readComments never exits, since the only condition to exit is on snap.ErrNoSnapshot, which will not happen as commitC <- nil only happens once while there exists snapshots.

This PR fixes this 2 issues by: differentiating initial load (where snapshot is immediately loaded instead of later), and directly return on receiving nil from commitC, instead of waiting indefinitely.

This fix is not very stable since there might be more strange issues with etcd's Raft API. The offending code is thus highlighted in case we have future issues here.