Simplify length computations / protect from wraparounds

[JelleAalbers]

For late records in pulses longer than 32767 samples (327.67 us at 10 ns, 65.5 us at 2ns resolution), the computation

len(record['data']) * record['record_i']

will result in an integer wraparound to negative values, since the record_i field is a 16 bit integer.

For the situations I tested, it seems this does NOT happen in numba compiled functions, only in pure numpy/python. So most of strax is safe. In any case, such long pulses should be pretty rare.

Nonetheless, I went through the code and removed places where we rely on this computation. There were several places where we did min(samples_per_record, pulse_length - record_i * record_length), but this is identically equal to length.

I also removed an unused and non-working data reduction function (replace_with_spike).