Not sure if this is optimization is necessary, but anyway I have some spare time so here it is. This replaces the slow modulo operation with a much faster 64bit multiplication & shift. This works when the hash is uniformly distributed between 0 and 2^32-1. This speeds up the benchmark by a factor of about 1.3:

```
RollingBloom, 5, 1500000, 3.73733, 4.97569e-07, 4.99002e-07, 4.98372e-07 # before
RollingBloom, 5, 1500000, 2.86842, 3.81630e-07, 3.83730e-07, 3.82473e-07 # FastMod
```

Be aware that this changes the internal data of the filter, so this should probably
not be used for CBloomFilter because of interoperability problems.

make check
src/bench/bench_bitcoin -filter=RollingBloom

Benchmark, evals, iterations, total, min, max, median
RollingBloom, 5, 1500000, 5.16318, 6.61227e-07, 7.3991e-07, 6.70607e-07

Benchmark, evals, iterations, total, min, max, median
RollingBloom, 5, 1500000, 3.73982, 4.92548e-07, 5.10237e-07, 4.95271e-07