This change makes the etcd package compatible with the existing Go
ecosystem for module versioning.
Used this tool to update package imports:
https://github.com/KSubedi/gomove
Several goroutines may call setPrevRev concurrently with different
revisions, all higher than prevRev. Previously all of these goroutines
could set prevRev, so prevRev may be replaced by older one.
If response's revision equals to prevRev, there's no need to call
setPrevRev.