Beginning of periods

[egor-ryashin]

See #3135

Benchmarks:

BenchmarkTimeSeries_hourly-10                             	      34	  30222012 ns/op	 1491715 B/op	   32548 allocs/op
BenchmarkTimeSeries_daily-10                              	      42	  26405661 ns/op	  171519 B/op	    3877 allocs/op
BenchmarkTimeSeries_weekly-10                             	      46	  26042627 ns/op	  123206 B/op	    2822 allocs/op
BenchmarkTimeSeries_weekly_first_day_of_week_Monday-10    	      44	  25638063 ns/op	  123120 B/op	    2822 allocs/op
BenchmarkTimeSeries_weekly_first_day_of_week_Sunday-10    	      45	  26494331 ns/op	  123498 B/op	    2824 allocs/op