Maxwell Tuning + perft_bb_gpu_simple

1. Use constant memory for lookup tables instead of texture memory. +10%
perf on Maxwell.
2. turn off USE_BITWISE_MAGIC_FOR_CASTLE_FLAG_UPDATION. Little bit
faster for maxwell.
3. use plain magics instead of fancy.
4. change thread block size to 192 from 256
5. Add perft_bb_gpu_simple() routine which has a single driver kernel
launching child kernels one at a time for each level. This is faster
(and much simpler) than recursive kernel launches on both Maxwell and
Kepler.
6. TODO: similar approach for 'hash' versions too (and maybe get rid of
all old code).