  * Replaced a few tabs with spaces for indentation.
  
  * Improve startup speed by creating separate index files for the shards
    while optimizing. (issue #75)
    
  * Fixed accidentally removing a shard from the index by the optimize thread.
    This was very unlikely to happen but technically a shard could be dropped
    and been re-created between checking the shard flags and locking the 
    shards mutex for replacing the shard in the index. 