*: enforce max lease TTL with 9,000,000,000 seconds

[yudai]

math.MaxInt64 / time.Second is 9,223,372,036. 9,000,000,000 is easier to
remember/document.

Closes #9374.

[yudai]

Do we need to copy the same value to the clientv3 package, so that client users can access the value easier?

[gyuho]

@yudai Sure. I would rename https://github.com/coreos/etcd/blob/master/clientv3/grpc_options.go to options.go, and define there as a constant?

[gyuho]

Also we need update https://github.com/coreos/etcd/blob/master/CHANGELOG-3.4.md#fixed-v3.

Thanks @yudai !

[yudai]

@gyuho done!

[gyuho]

@yudai Thanks! I will merge this by today.