Fix deadlock between heads and headmtx
Fix deadlock between heads and headmtx

With hundreds of concurrent appenders the locking order between the
headBlocks on instantiating appenders and write locking the headmtx
is hard to impossible to get consistent.
Just never instantiate appenders while holding the headmtx lock in any
way.

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