perf: [pkg/timeseries/dataset] refactor start and end calculation

[crandles]

1. Remove 2 extra goroutines from being created per usage of the pkg/timeseries/dataset cropper(s).

Instead of looking for the start and end time in two separate goroutines, look for the start and then end time.

This is already itself in a goroutine, the extra concurrency overhead at scale might be working against us with this particular usage.

2. Moves away from custom binary search to using stdlib.

[coveralls]

Pull Request Test Coverage Report for Build 20322675914

Details

• 22 of 22 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 72.114%

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Totals
Change from base Build 20322313354:	-0.03%
Covered Lines:	14911
Relevant Lines:	20677

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💛 - Coveralls