fix: better jitter for fractional index

[Leeeon233]

If jitter is enabled, random bytes with jitter length will be appended to FractionalIndex.

If you want 99% probability of no collision:

k = 3, jitter = 1 k = 37, jitter = 2 k = 582, jitter = 3 k = 9292, jitter = 4 k = 148665, jitter = 5

k is the number of parallel operations at the same position