varz: export uptime in a custom header

This can be used to detect that the process was restarted.
skip full.idx (prevents (harmless) extra garbage collection)
Bugfix: don’t create index files with .idx suffix
varz-to-influxdb: detect restarts, fix counters, add seconds-since-now
dcs-web: log finished queries to InfluxDB

We cannot easily export this via the varz interface, so we push points
into this timeseries from dcs-web.
varz: export the amount of available bytes on /dcs-ssd
initialize counter varz to 0, otherwise we cannot plot rarely-increased counters

(Because before they are incremented, they don’t exist yet in InfluxDB,
so the graphs will not render.)
feeder: export more counters (garbage collection, sanity-check feeding)