rocksdb version metaserver is very slow to start loading data

[YunhuiChen]

Describe the bug (描述bug)
rocksdb version metaserver is very slow to start loading data

To Reproduce (复现方法)
1、mdtest creates a lot of files, around a few million
2、restart metaserver
3、copyset has been loaded for 15 hours and still not finished

Expected behavior (期望行为)

Versions (各种版本)
OS:
Compiler:
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Additional context/screenshots (更多上下文/截图)