Let the broker inform proxies how often to poll

Currently, proxies poll the broker at a static rate of once every 5–10 seconds. If we're anticipating thousands of proxies, we don't need them to poll so frequently.

The broker could instead tell each proxy how long to wait before polling again. The broker could even dynamically adjust the rate based on an estimate of supply and demand.

One way to do this would be a custom header in responses to /proxy requests:

Snowflake-Next-Poll: Thu, 22 Mar 2018 18:05:47 GMT

Or using a relative time offset:

Snowflake-Next-Poll: 600

There was a similar idea for flash proxy. legacy/trac#8171 (closed):: The facilitator included a fixed check-back-in=600 in its responses. legacy/trac#8172 (closed):: Adjust polling interval dynamically (never implemented).