Support more efficient way to persist manifest of TimeMergeStorage

[baojinri]

Describe This Problem

Now, when meta information of TimeMergeStorage changes, the efficiency of persisting manifest sst files is very low. Each time, the new sst file will be added to the manifest and then persisted it whole.

Proposal

Each time, the new sst file will be persisted instantly, then add the new sst into snapshot background.
The manifest file is organized as follows:

/root/manifest/snapshot
/root/manifest/new_sst_0
/root/manifest/new_sst_1
/root/manifest/new_sst_2
...

Additional Context

Process manifest files：

• When initializing the manifest, we will list all the sst files in the manifest directory and then add them into the snapshot
• Every time a new file is added, we will count the new files. When the number of new files reaches 50, we will list all the sst files in the manifest directory and then add them into the snapshot