[Feature] Support Hybrid Checkpoint Trigger (Time Interval + Dirty Page Ratio)

[zhangh43]

Description:
Introduce a hybrid checkpoint trigger mechanism that initiates a checkpoint when either of the following conditions is met:

1. A configurable time interval has elapsed
2. A configurable dirty page ratio threshold is exceeded

This design avoids triggering checkpoints that flush only a small amount of dirty data, which can cause write amplification, especially in EloqKV’s architecture with many partitions (≈1000) and small per-file size (8 MB).

Background / Motivation:

• Frequent time-based checkpoints may flush very small dirty sets

• Small flushes across many partitions increase:

  • Write amplification
  • Cloud storage I/O cost

Acceptance Criteria:

• Checkpoints trigger when either condition is met