Adjust proxy poll rate to match increased client load

Looking at recent stats from the broker, we're starting to get situations where clients are polling for proxies and not getting any available ones. Here's a visualization of data from September and October:

It looks like the number of proxies has stayed about the same for several months, around 6000:

But our client poll rates have gone up substantially.

I suggest decreasing both the defaultBrokerPollInterval and the pollAdjustment. We might want to get rid of the latter eventually since it didn't seem to help at all with snowflake#33666 (closed)