compute new first_block_timestamp once
compute new first_block_timestamp once

first_block_timestamp doesn't change across granularities or
aggregates. let's compute it once rather than granularities*aggregates
times.

Change-Id: If03132e6c96976f993847965645999d547f85698

gnocchixyz/gnocchi,leandroreox/gnocchi,leandroreox/gnocchi,sileht/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi