LCGDM-2988 allow metadata cache reconfiguration
Refresh DOME metadata cache configuration with dome_config interface. This can be used to drop records from metadata cache without restarting dmlite process - just configure short cache expiration time and wait for cleanup task, e.g.
$ dmlite-shell -e 'config set head.mdcache.itemttl 5'
$ # wait glb.tickfreq time (default 5s)
$ dmlite-shell -e 'config set head.mdcache.itemttl 7200'