Ganglia logtailer missing as zero

Add a flag -z which , when present, remembers metrics that are submitted across cron runs and always reports all metrics it's ever seen. This fixes a bug where, when creating metrics based on log file content, metrics that disappear from the log file appear as a flat line instead of 0.
For example, if you're generating metric names using the HTTP response code and you rarely get 503 errors, when you first get a spurt of errors they'll get reported but as soon as the issue is resolved the last known value will remain instead of reporting 0 503 errors. This is similar to setting dmax appropriately, except that instead of the 503 metric disappearing from ganglia, it remains present with a value of zero.