Stop truncating microsecond everywhere with a util package

Created by: unknwon

Currently, we have many places that do .Truncate(time.Microsecond) that truncates the microsecond part of a timestamp because Postgres 9.6 does not support saving microsecond. Tests would fail if we don't truncate and compare time values between Go and what's read from DB.

We should have a timeutil or clock package as a wrapper to take care of this.

Context: https://github.com/sourcegraph/sourcegraph/pull/11136/files#r433723412