[mtp] Increase thumbnail capacity

The thumbnailer client was arbitrarily limited to 50 open thumbnail requests.
This was too little when dealing with a directory of 250 photos.
I guess it's good to have *some* limit, as a safeguard against running
out of memory.
Arbitrarily increased it to 2000.
Merge pull request #19 from amtep/master

[mtp] Increase thumbnail capacity