Compare not equal operator is broken in etcd

I couldn't believe it myself, and after a long day of hacking, I finally assumed it was something in etcd's code that was broken instead of mine. Here's a simple reproducer and logs.

What are we using?
```
$ etcd --version
etcd Version: 3.3.12
Git SHA: d57e8b8d9
Go Version: go1.10.8
Go OS/Arch: linux/amd64
```

Run etcd in an empty dir:
```
etcd
2019-03-20 14:49:36.168882 I | etcdmain: etcd Version: 3.3.12
2019-03-20 14:49:36.168944 I | etcdmain: Git SHA: d57e8b8d9
2019-03-20 14:49:36.168955 I | etcdmain: Go Version: go1.10.8
2019-03-20 14:49:36.168963 I | etcdmain: Go OS/Arch: linux/amd64
2019-03-20 14:49:36.168971 I | etcdmain: setting maximum number of CPUs to 4, total number of available CPUs is 4
2019-03-20 14:49:36.168982 W | etcdmain: no data-dir provided, using default data-dir ./default.etcd
2019-03-20 14:49:36.169433 I | embed: listening for peers on http://localhost:2380
2019-03-20 14:49:36.169547 I | embed: listening for client requests on localhost:2379
2019-03-20 14:49:36.169939 I | etcdserver: name = default
2019-03-20 14:49:36.169953 I | etcdserver: data dir = default.etcd
2019-03-20 14:49:36.169963 I | etcdserver: member dir = default.etcd/member
2019-03-20 14:49:36.169968 I | etcdserver: heartbeat = 100ms
2019-03-20 14:49:36.169972 I | etcdserver: election = 1000ms
2019-03-20 14:49:36.169979 I | etcdserver: snapshot count = 100000
2019-03-20 14:49:36.169993 I | etcdserver: advertise client URLs = http://localhost:2379
2019-03-20 14:49:36.170001 I | etcdserver: initial advertise peer URLs = http://localhost:2380
2019-03-20 14:49:36.170014 I | etcdserver: initial cluster = default=http://localhost:2380
2019-03-20 14:49:36.179266 I | etcdserver: starting member 8e9e05c52164694d in cluster cdf818194e3a8c32
2019-03-20 14:49:36.179303 I | raft: 8e9e05c52164694d became follower at term 0
2019-03-20 14:49:36.179319 I | raft: newRaft 8e9e05c52164694d [peers: [], term: 0, commit: 0, applied: 0, lastindex: 0, lastterm: 0]
2019-03-20 14:49:36.179328 I | raft: 8e9e05c52164694d became follower at term 1
2019-03-20 14:49:36.180643 W | auth: simple token is not cryptographically signed
2019-03-20 14:49:36.181095 I | etcdserver: starting server... [version: 3.3.12, cluster version: to_be_decided]
2019-03-20 14:49:36.181467 I | etcdserver: 8e9e05c52164694d as single-node; fast-forwarding 9 ticks (election ticks 10)
2019-03-20 14:49:36.182013 I | etcdserver/membership: added member 8e9e05c52164694d [http://localhost:2380] to cluster cdf818194e3a8c32
2019-03-20 14:49:37.179884 I | raft: 8e9e05c52164694d is starting a new election at term 1
2019-03-20 14:49:37.179965 I | raft: 8e9e05c52164694d became candidate at term 2
2019-03-20 14:49:37.180035 I | raft: 8e9e05c52164694d received MsgVoteResp from 8e9e05c52164694d at term 2
2019-03-20 14:49:37.180098 I | raft: 8e9e05c52164694d became leader at term 2
2019-03-20 14:49:37.180139 I | raft: raft.node: 8e9e05c52164694d elected leader 8e9e05c52164694d at term 2
2019-03-20 14:49:37.180583 I | etcdserver: setting up the initial cluster version to 3.3
2019-03-20 14:49:37.180998 N | etcdserver/membership: set the initial cluster version to 3.3
2019-03-20 14:49:37.181059 I | embed: ready to serve client requests
2019-03-20 14:49:37.181239 I | etcdserver/api: enabled capabilities for version 3.3
2019-03-20 14:49:37.181303 I | etcdserver: published {Name:default ClientURLs:[http://localhost:2379]} to cluster cdf818194e3a8c32
2019-03-20 14:49:37.181539 E | etcdmain: forgot to set Type=notify in systemd service file?
2019-03-20 14:49:37.182692 N | embed: serving insecure client requests on 127.0.0.1:2379, this is strongly discouraged!
```

Here's a golang example to trigger the issue:

```
package main

import (
"context"
"log"

etcd "github.com/coreos/etcd/clientv3"
)

func main() {
log.Printf("hello")
cfg := etcd.Config{
Endpoints: []string{"http://127.0.0.1:2379"},
// 0 disables auto-sync. By default auto-sync is disabled.
AutoSyncInterval: 0,
}
client, err := etcd.New(cfg) // connect!
if err != nil {
log.Fatal(err)
}
log.Printf("connected")

ifs := []etcd.Cmp{}
ops := []etcd.Op{}
//els := []etcd.Op{}

key := "/_hello/foo"
data := "world"

// works
//ifs = append(ifs, etcd.Compare(etcd.Value(key), "=", data)) // desired state
//els = append(ops, etcd.OpPut(key, data)) // TODO: add a TTL? (etcd.WithLease)

ifs = append(ifs, etcd.Compare(etcd.Value(key), "!=", data)) // desired state
ops = append(ops, etcd.OpPut(key, data)) // TODO: add a TTL? (etcd.WithLease)

log.Printf("txn1...")
//resp1, err := client.KV.Txn(context.TODO()).If(ifs...).Then([]etcd.Op{}...).Else(els...).Commit()
resp1, err := client.KV.Txn(context.TODO()).If(ifs...).Then(ops...).Else([]etcd.Op{}...).Commit()
log.Printf("result1: %+v || %+v", resp1, err) // succeeded is set to true if the compare evaluated to true

log.Printf("txn2...")
//resp2, err := client.KV.Txn(context.TODO()).If(ifs...).Then([]etcd.Op{}...).Else(els...).Commit()
resp2, err := client.KV.Txn(context.TODO()).If(ifs...).Then(ops...).Else([]etcd.Op{}...).Commit()
log.Printf("result2: %+v || %+v", resp2, err) // succeeded is set to true if the compare evaluated to true

// test with: ETCDCTL_API=3 etcdctl get / --prefix
}
```

When the golang code is run, the following appears in the etcd server logs:

```
proto: no coders for int
proto: no encoder for ValueSize int [GetProperties]
proto: no coders for int
proto: no encoder for ValueSize int [GetProperties]
```

And running etcdctl produces no output:

```
$ ETCDCTL_API=3 etcdctl get / --prefix
```

If you modify the above golang example to use `=` instead of `!=`, and switch the `then` for the `else` case, then it works correctly as expected.

Thoughts welcome!

Thanks

Start with the clientv3 transaction path used by etcd.KV.Txn, Compare, and Value, then reproduce the issue with the Go example against etcd 3.3.12. Check the transaction result and the reported proto encoder logs, and compare the != behavior with the working = case. Done means the comparison evaluates as expected without the reported errors.

Written by the indexing model from the issue text.