loggregator-agent should evenly distribute traffic across Dopplers

[pianohacker]

Expected behavior

loggregator-agents should distribute downstream traffic among 5 downstream Dopplers, rotating those Dopplers on a reasonable cadence.

Actual behavior

The loggregator-agents will only rotate Dopplers every 100,000 batches (up to 10 million envelopes, depending on batching). Also, during upgrades or other circumstances, they can end up sending traffic to only one downstream Doppler.

[cf-gitbot]

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