etcdserver: record removed member to check incoming message

I put removed member in a new directory other than members because 1. removed members has no need to record raftAttributes and attributes anymore 2. removed members have different use case from active members, and they don't need to be fetched at the same time
The naming in the interface is getting messy:
probably this is cleaner
```go
GetCluster() Cluster
AddMember(m Member)
RemoveMember(id uint64)
IsRemovedMember(id uint64) bool
```