Allow to compute sequence stats more than once

Since Hive does not allow to delete rows, we need to partition the
table, so we can remove computed stats by dropping partitions.

Since the different webrequest_sources are different pipelines, we not
only partition per hour, but also per webrequest_source, so we can
cover each pipeline individually.

Change-Id: Ia48d287920961acf7d80c629e357bf321ab9a2e4