Precalculate memSeries.head
Precalculate memSeries.head

This is read far more than it changes.
This cuts ~14% off walltme and ~27% off CPU for WAL reading.

Signed-off-by: Brian Brazil <aadf89994a343dc176a47081f0c0bbea26f0a345@robustperception.io>

GoogleCloudPlatform/prometheus,alileza/prometheus,prometheus/tsdb,prometheus/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,alileza/prometheus,kangwoo/prometheus,Gouthamve/prometheus,prometheus/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,Gouthamve/prometheus,kangwoo/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,Gouthamve/tsdb,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,alileza/prometheus,alileza/prometheus,kangwoo/prometheus,prometheus/prometheus,kangwoo/prometheus