Improve the names of a few commonly used APIs

[glbrntt]

Motivation:

Naming is important; it should be clear and concise.

Modifications:

The follow renames all offer more precise names:

• Rename server.run() to server.serve()
• Rename server.stopListening() to server.beginGracefulShutdown()
• Rename client.close() to client.beginGracefulShutdown()

Result:

Clearer APIs