Improve commit load balancing

The current load balancing algorithm for sending commits to proxies is completely random. This leads to high tail latencies when a lot of data is all sent to the same proxy.

A better strategy would be to track the amount of bytes each client has currently outstanding to each proxy, and always send the next commit to the least loaded proxy.

The interaction with https://github.com/apple/foundationdb/pull/2520 needs to be considered. Since some proxies will have more client connections, the balancing should not be completely even.