Currently Tablet meta checkpoint is a memory-exhausted operation.
If a host has 12 disks, it will start 12 threads to do tablet meta checkpoint.
In our experience, the data size of one tablet can be as high as 2G.
If 12 threads do the checkpoint at the same time, it maybe cause OOM.
Therefore, this PR try to solve this problem.
Firstly, it only start one thread to produce table meta checkpoint tasks.
Secondly, it creates a thread pool to handle these tasks.
You can configure the size of the thread pool to control the parallelism in case of OOM.
It is a producer-customer model.
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