Round down the task start time when checking the report timestamp

Aggregators are meant to reject reports with timestamps that precede the
task start time. But the Client is also supposed to round down its
timestamp, meaning it may get rejected even if upload time is valid.

To fix this, round the task start time before comparing to the report
timestamp.

draft-ietf-ppm-dap.md

@@ -1881,9 +1881,11 @@ following checks:
    Aggregator SHOULD mark the input share as invalid with error
    `report_too_early`.
 
-1. Check if the report's timestamp is before the task's `task_start` time. If
-   so, the Aggregator MUST mark the input share as invalid with the error
-   `task_not_started`.
+1. Check if the report's timestamp is before the task's start time. If the
+   timestamp is less than `task_start - (task_start % time_precision)`, the
+   Aggregator MUST mark the input share as invalid with the error
+   `task_not_started`. Note that the task start time is rounded down since the
+   Client may have rounded down its timestamp as well.
 
 1. Check if the report's timestamp is past the task's end time, given by
    `task_start + task_duration`. If so, the Aggregator MUST mark the input