consistenthash: replace linear search with binary search

The binary search quickly out-paces the linear search, even for a small number
of shards and replicas.

benchmark          old ns/op    new ns/op    delta
BenchmarkGet8           122          122   +0.00%
BenchmarkGet32           471          137  -70.91%
BenchmarkGet128         5619          254  -95.48%
BenchmarkGet512       90302          406  -99.55%