Defrag: Allow to set target extent size

[yennor]

I've had a DB file with 500 000 extends which just never got defraged. After looking into it, I found that the defrag command by default uses -t 32m .
After doing the defrag with a lot bigger -t value, I managed to get the extends down to 5000.
So it would be cool if we could set the -t value in the config.