Fix compacting disable/enable
Fix compacting disable/enable

Enabling and disabling compaction no longer blocks are potentially
causes panics.

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