add Writer.close() and Client.close(), improve Reader.close()

[ploxiln]

make Reader.close() prevent nsqd_tcp_addresses from reconnecting

Writer.close() also closes conns and stops from reconnecting

Client.close() stops the periodic _check_last_recv_timestamps()

fixes #196

[mreiferson]

These changes seem fine, ready to land this @ploxiln?

[ploxiln]

unfortunately I haven't tested this at all, I've just been using the tornado-6 branch