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%
Merge pull request #26 from dgryski/consistenthash-binary-search

consistenthash: replace linear search with binary search