Performance optimized interleaved mode JetStream server

[JoeZijunZhou]

• Optimized TPU duty cycle (largest gap < 4ms)
• Optimized TTFT: dispatch prefill tasks ASAP w/o unnecessary blocking in CPU, keep backpressure to enforce insert ASAP, return first token ASAP.
• Optimized TPOT: properly enforce generate and detokenize task in sequential w/o unnecessary blocking in CPU.
• Optimized output token throughput: properly prioritize prefill and balancing TTFT and decode in high throughput situation.
• Tested with llama2-70b JetStream MaxText server on v5e-8 VM