Scatter might have a skewed distribution if the operators fails

[rleungx]

Enhancement Task

We use a TTL cache to calculate the distribution of scatter, see https://github.com/tikv/pd/blob/d6d9feab3e2a5180acbcc7095723d43e97798686/pkg/schedule/scatter/region_scatterer.go#L68

But if the operator is created but fails, the cache won't be changed. If the client retries, it might have a skewed distribution.