Reuse some big.Rat in getPlannedRateChanges
Reuse some big.Rat in getPlannedRateChanges

name                     old time/op    new time/op    delta
GetPlannedRateChanges-4    1.66ms ± 7%    1.34ms ± 2%  -19.42%  (p=0.008 n=5+5)

name                     old alloc/op   new alloc/op   delta
GetPlannedRateChanges-4    1.22MB ± 0%    0.86MB ± 0%  -29.13%  (p=0.008 n=5+5)

name                     old allocs/op  new allocs/op  delta
GetPlannedRateChanges-4     51.6k ± 0%     37.2k ± 0%  -27.86%  (p=0.008 n=5+5)

loadimpact/k6,loadimpact/k6