When running 100 times in row those tests flaked around 10-20%. Based on
some experimentation 10 keys was enough to ensure that wal snapshot is
created and prevented any flakes.
Thanks to this, unix sockets should be not longer
created by integration tests in the the source code directory,
so potentially trigger IDE reloads and unnecessery load (and mess).
"snapshot" Restore/Status code was the only remaining dependency of client on 'server'
code. The code is solelly used by etcdctl. Long-term the snapshot code
should be migrated to 'etcdadm' style of tool such that we can
distinguish tool solelly depending on networking API vs. tools that
operation on etcd files directly.
We left snapshot.Save() code in clientv3.snapshot package, such that
clients can benefits from automated download&safe to file snapshot
functionality over the wire.