Set default binary-caches-parallel-connections to 25

Some benchmarking suggested this as a good value. Running

  $ benchmark -f ... -t 25 -- sh -c 'rm -f /nix/var/nix/binary-cache*; nix-store -r /nix/store/x5z8a2yvz8h6ccmhwrwrp9igg03575jg-nixos-15.09.git.5fd87e1M.drv --dry-run --option binary-caches-parallel-connections <N>'

gave the following mean elapsed times for these values of N:

N=10:  3.3541
N=20:  2.9320
N=25:  2.6690
N=30:  2.9417
N=50:  3.2021
N=100: 3.5718
N=150: 4.2079

Memory usage is also reduced (N=150 used 186 MB, N=25 only 68 MB).

Closes #708.