Perf improvements with concurrent hash calculation
=================================================

Using the source code dataset here are the new perfs:

```
        254 541,85 msec task-clock                #    1,333 CPUs utilized          
           489 390      context-switches          #    0,002 M/sec                  
            14 491      cpu-migrations            #    0,057 K/sec                  
           109 170      page-faults               #    0,429 K/sec                  
   702 598 342 141      cycles                    #    2,760 GHz                    
 1 191 229 091 705      instructions              #    1,70  insn per cycle         
   172 579 644 365      branches                  #  678,001 M/sec                  
     2 502 920 412      branch-misses             #    1,45% of all branches        

     191,024430360 seconds time elapsed

     247,992304000 seconds user
      15,759037000 seconds sys
```


But this time I also had the good idea to close all my processes and to use
a tmp directory for writing.

-------------------------------------------------------------------------------

With the same setup, the previous perf was:

```
        277 665,78 msec task-clock                #    1,411 CPUs utilized          
           853 639      context-switches          #    0,003 M/sec                  
            27 276      cpu-migrations            #    0,098 K/sec                  
           110 187      page-faults               #    0,397 K/sec                  
   764 443 227 093      cycles                    #    2,753 GHz                    
 1 221 696 199 089      instructions              #    1,60  insn per cycle         
   178 891 873 274      branches                  #  644,271 M/sec                  
     2 578 200 052      branch-misses             #    1,44% of all branches        

     196,744991354 seconds time elapsed

     270,030535000 seconds user
      18,285378000 seconds sys
```

So not that big of an improvement, but it seems that at the same time CPU usage
has decreased a bit. Maybe because less synchronization calls were made ?