[MySQL] Separate sequencing and integration

[roger2hk]

#190

Local hammer test result:

┌────────────────────────────────────────────────────────────────────────┐
│Read (228 workers): Current max: 0/s. Oversupply in last second: 0      │
│Write (622 workers): Current max: 12978/s. Oversupply in last second: 0 │
│TreeSize: 609645 (Δ 10113qps over 30s).                                 │
│Time-in-queue: 15ms/20ms/25ms (min/avg/max)                             │
│Observed-time-to-integrate: 1005ms/1010ms/1017ms (min/avg/max)          │
└───────────────────────────────────────────────────────────────────────