Align timeseries buckets

[xurui-c]

Fixes https://github.com/getsentry/projects/issues/364 by making start_timestamp divisible by granularity. This makes it so that our timebuckets line up properly.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
2689	1	2688	5

View the top 1 failed tests by shortest run time

tests.web.rpc.v1.test_endpoint_time_series.TestTimeSeriesApi::test_rachel

Stack Traces | 0.91s run time

Traceback (most recent call last):
  File ".../rpc/v1/test_endpoint_time_series.py", line 269, in test_rachel
    assert len(response.result_timeseries) != 0
AssertionError: assert 0 != 0
 +  where 0 = len([])
 +    where [] = meta {\n  request_id: "0e6f016e-047f-43fd-88b6-98be655cf769"\n}\n.result_timeseries

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