Set the number of dist_threads to 7

http://notes.jschutz.net/2011/03/sphinx-search-engine-comparative-benchmarks/

I found an interesting article ("see above") about Sphinx performance.

It seems that the best performance is achieved when 

dist_threads = number of indices

So I tried to change that to 7 (7 different search indices) and found out the performance really improved.

I was able to perform 492 random requests on test in 
66 seconds
while it was taking around
76 seconds
before that change.

I think it is a good improvement. (You can find the script on mf1t in my sandbox)