Execution Control currently exposes cumulative queue wait time, which only supports average wait-time calculations. Add bucketed wait-time histograms for EC queue acquisitions so downstream metrics can compute p95/p99 wait times over a time window. Expose the histogram buckets in serverStatus/FTDC for read/write and priority queues.

Possible implementation:
When an operation finishes, it increments the right latency bucket, with a possible distribution like

0µs, 10µs, 25µs, 50µs, 100µs, 250µs, 500µs,
1ms, 2.5ms, 5ms, 10ms, 25ms, 50ms,
100ms, 250ms, 500ms, 1s, 2.5s, 5s, 10s, 30s+