Consider binary search in Histogram aggregator

[jmacd]

See this comment: https://github.com/open-telemetry/opentelemetry-go/pull/438/files#r370398569

There is a question of whether we will support alternative kinds of bucket range schemes, such as linear and logarithmic, both of which have important special cases. In general, when the number of buckets exceeds a small threshold, it's better to use binary search when one of these alternates does not apply.