This also happens without gRPC proxy.

Fix panic when gRPC proxy leader watcher is restored:

```
go test -v -tags cluster_proxy -cpu 4 -race -run TestV3WatchRestoreSnapshotUnsync

=== RUN   TestV3WatchRestoreSnapshotUnsync
panic: watcher minimum revision 9223372036854775805 should not exceed current revision 16

goroutine 156 [running]:
github.com/coreos/etcd/mvcc.(*watcherGroup).chooseAll(0xc4202b8720, 0x10, 0xffffffffffffffff, 0x1)
	/home/gyuho/go/src/github.com/coreos/etcd/mvcc/watcher_group.go:242 +0x3b5
github.com/coreos/etcd/mvcc.(*watcherGroup).choose(0xc4202b8720, 0x200, 0x10, 0xffffffffffffffff, 0xc420253378, 0xc420253378)
	/home/gyuho/go/src/github.com/coreos/etcd/mvcc/watcher_group.go:225 +0x289
github.com/coreos/etcd/mvcc.(*watchableStore).syncWatchers(0xc4202b86e0, 0x0)
	/home/gyuho/go/src/github.com/coreos/etcd/mvcc/watchable_store.go:340 +0x237
github.com/coreos/etcd/mvcc.(*watchableStore).syncWatchersLoop(0xc4202b86e0)
	/home/gyuho/go/src/github.com/coreos/etcd/mvcc/watchable_store.go:214 +0x280
created by github.com/coreos/etcd/mvcc.newWatchableStore
	/home/gyuho/go/src/github.com/coreos/etcd/mvcc/watchable_store.go:90 +0x477
exit status 2
FAIL	github.com/coreos/etcd/integration	2.551s
```

gRPC proxy spawns a watcher with a key "proxy-namespace__lostleader"
and watch revision "int64(math.MaxInt64 - 2)" to detect leader loss.
But, when the partitioned node restores, this watcher triggers
panic with "watcher minimum revision ... should not exceed current ...".

This check was added a long time ago, by my PR, when there was no gRPC proxy:

https://github.com/coreos/etcd/pull/4043#discussion_r48457145

> we can remove this checking actually. it is impossible for a unsynced watching to have a future rev. or we should just panic here.

However, now it's possible that a unsynced watcher has a future
revision, when it was moved from a synced watcher group through
restore operation.

This PR adds "restore" flag to indicate that a watcher was moved
from the synced watcher group with restore operation. Otherwise,
the watcher with future revision in an unsynced watcher group
would still panic.

Example logs with future revision watcher from restore operation:

```
{"level":"info","ts":1527196358.9057755,"caller":"mvcc/watcher_group.go:261","msg":"choosing future revision watcher from restore operation","watch-key":"proxy-namespace__lostleader","watch-revision":9223372036854775805,"current-revision":16}
{"level":"info","ts":1527196358.910349,"caller":"mvcc/watcher_group.go:261","msg":"choosing future revision watcher from restore operation","watch-key":"proxy-namespace__lostleader","watch-revision":9223372036854775805,"current-revision":16}
```

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>