NameResolver can be NotThreadSafe

[zhangkun83]

With ManagedChannelImpl2, NameResolver is always called from Channel Executor, except for getAuthority(). After ManagedChannelImpl2 is promoted, NameResolver can get rid of @ThreadSafe.

[zhangkun83]

As pointed out in #2918, we would need to expose Channel Executor to the NameResolver, maybe similar to LoadBalancer.Helper.runSerialized(). Otherwise if NameResolver run a task asynchronously, it can't serialize that task with Channel Executor, thus would require other means of synchronization, which beats the purpose of making NameResolver NotThreadSafe.