carbonara: optimize _first_block_timestamp
carbonara: optimize _first_block_timestamp

Leverage _round_timestamp which is a lot faster than faking a resample
using Pandas.

Change-Id: I05709a728f1605227a80e2347a9436fc6bfcf832

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