This PR implements chunked sending/chunked loading of binaries in Perspective's websocket implementations in both Python and Javascript. Apache Arrow binaries above a certain size will be sent to the Perspective client in chunks, which prevents the Websocket from locking on a particularly large binary and closing due to a timeout, as once that binary is on the wire all heartbeat messages/other messages are blocked from being sent/received.

The default chunk threshold is configured to be 200MB, and arrows will be transported in 50MB chunks. In Python, these settings can be altered in the PerspectiveManager constructor by initializing a new PerspectiveManager with chunk_threshold and chunk_size, which are defined in bytes:

manager = PerspectiveManager(
    chunk_threshold=500 * 1000 * 1000,  # in bytes
    chunk_size=100 * 1000 * 1000
)

In NodeJS, these settings can be altered on the WebsocketManager:

const manager = new WebsocketManager();
manager.chunk_threshold = 500 * 1000 * 1000;  // in bytes
manager.chunk_size = 100 * 1000 * 1000;

Additionally, this PR implements a client-level heartbeat message (ping/pong) between Perspective client and server, which should aid in debugging as heartbeat messages now show up in the browser debug console. In Tornado, servers previously set up with websocket_ping_interval should remove their websocket_ping_interval setting and depend on the client-level heartbeats to keep the Perspective websocket connection alive.

Changelog

• Implements chunked binary sending on PerspectiveTornadoServer in Python and WebSocketManager in JS
• Implements chunked binary receiving in WebSocketClient in JS and PerspectiveTornadoClient in Python
• Adds tests for chunked send/receive in Python
• Adds ping/pong heartbeat for Websockets