Improve the iterate seek to O(logN)

[nolouch]

Hi, In our scenario, we frequently use the iterate, and the Less compare is expenses when key is large. This PR mainly improve the iterate location. uses binary search to quickly locates the start item.
before:

BenchmarkSeek-4          3000000               452 ns/op
BenchmarkSeek-4          3000000               513 ns/op
BenchmarkSeek-4          3000000               454 ns/op
BenchmarkSeek-4          3000000               450 ns/op

after:

BenchmarkSeek-4         10000000               225 ns/op
BenchmarkSeek-4         10000000               222 ns/op
BenchmarkSeek-4         10000000               223 ns/op
BenchmarkSeek-4         10000000               224 ns/op

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[googlebot]

CLAs look good, thanks!

[nolouch]

PTAL @gconnell

[gconnell]

Thanks!