Computing the snapshot file path is error prone; snapshot recovery was
constructing file paths missing a path separator so the snapshot
would never be loaded. Instead, refactor the backend path handling
to use helper functions where possible.
This commit lets etcdserver skip needless log entry applying. If the
result of log applying isn't required by the node (client that issued
the request isn't talking with the node) and the operation has no side
effects, applying can be skipped.
It would contribute to reduce disk I/O on followers and be useful for
a cluster that processes much serializable get.
