Have implicit activity context timeout start-to-close be relative to system time not server time

[cretz]

Describe the solution you'd like

Currently the code at

sdk-go/internal/activity.go

Lines 444 to 454 in a921890

	func calculateActivityDeadline(scheduled, started time.Time, scheduleToCloseTimeout, startToCloseTimeout time.Duration) time.Time {
	startToCloseDeadline := started.Add(startToCloseTimeout)
	if scheduleToCloseTimeout > 0 {
	scheduleToCloseDeadline := scheduled.Add(scheduleToCloseTimeout)
	// Minimum of the two deadlines.
	if scheduleToCloseDeadline.Before(startToCloseDeadline) {
	return scheduleToCloseDeadline
	}
	}
	return startToCloseDeadline
	}

has start-to-close calculated from server-provided start time instead of current system time which means it may be subject to skew (ref #1912). Have it based on time.Now() instead. Note, nothing can be done about schedule-to-start equivalent because there is no local-time equivalent for "scheduled".