Memory tuning on header synchronization

[chenyukang]

With the merge of the previous PR: #3391

We may get some memory issue because of spacejam/sled#1388

As discussed with @zhangsoledad , a possible solution is:

1. Replace sled. For this scenario, a btree database such as lmdb is more suitable. We are currently considering lmdb as a candidate, we need more tests and research.

2. Add the original small memory mode back as a configurable option.

3. Provide multiple configuration options to adapt to different scenarios, such as:

• Stable operation under low memory conditions.
• Fastest synchronization but with high memory usage.
• Balanced performance.