replace original file and rebuilt index after merge

[Jeremy-Run]

I have two ideas for this function:

1. After the merge is completed, replace the file and rebuild the index from disk.
   Advantages: Memory usage does not increase.
   Disadvantages: The whole process requires a write lock, and the db cannot be read or written externally.

2. Put the written index data into the temporary storage area during the merge process (like copy-on-write), and build an index while building the hint file. When the merge ends, the index of the temporary storage area and the hint index to combine.
   Advantages: DB's write lock time is smooth and can be merged smoothly.
   Disadvantage: There may be more than twice the memory overhead.

Based on the shortcomings of bitcase, I chose the first option.