wal: synchronize background operations
wal: synchronize background operations

This adds an actor channel and thereby serializes all background
operations through the run() method.
Fixes an existing race.

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