Creates a LevelDB snapshot.
The snapshot creation process first closes the LevelDB subsystem, then creates a directory under the
snapshots directory in the LevelDB data directory and then creates hard links to the SST files and the log, manifest and CURRENT files. Once those hard links are created, the LevelDB subsystem is reopen.
Since the snapshot mechanism works by creating hard links, it can only work if the
snapshots directory is on the same device (volume) as the LevelDB data directory.
The content of the newly created snapshot can be copied to third party storage for archival purposes.