Preserve Block Data in Bucket when Destroying a JuiceFS Partition

[michaelmyc]

What would you like to be added:

The ability to preserve object store bucket when destroying a JuiceFS partition.

Why is this needed:

In case the need to swap metadata engine, I can preferably do the following:

1. dump metadata from old DB
2. load metadata to new DB
3. juicefs destroy (while preserving block data in object store)
4. juicefs format

instead of:

1. dump metadata from old DB
2. mirror block data to new bucket
3. load metadata to new DB
4. juicefs destroy (destroying block data in the process)
5. juicefs format

[davies]

You can destroy the database (drop database xxx or flushdb xxx) directly without using juicefs destroy.