when installing / first running the plugin, it can take longer than
one munin-update call to create the $TIMEFILE. so until it is created
we assume the $LOCKFILE is not too old yet and let the first run finish
before we spawn additional processes.
Don't exit with error if cache file is missing, instead refrain from sending values and daemonize() in an attempt to generate one.
Invocation using 'config' before generation is successful will fail, as mentioned in #914, but configuration will get picked up as soon as it is present.
