Function reshard 

pub fn reshard(
    dir: &Path,
    shards: u32,
    opts: &ReshardOptions,
) -> Result<Resharded>

Changes the shard count of the local data directory at dir to shards. The directory must be closed: this takes its lock. It must be in log format 3 (fusee upgrade converts an older one). Also finishes a reshard that was interrupted after its switch began.