We lost the nice context-stack [jobname][taskname][...] at the beginning
of each log line when switching to logrus.
Define some field names that define these contexts.
Write a human-friendly formatter that presents these field names like
the solution we had before logrus.
Write some other formatters for logfmt and json output along the way.
Limit ourselves to stdout logging for now.
Implement
* pruning on source side
* local job
* test subcommand for doing a dry-run of a prune policy
* use a non-blocking callback from autosnap to trigger the depending
jobs -> avoids races, looks saner in the debug log
Don't use jobrun for daemon, just call JobDo() once, the job must
organize stuff itself.
Sacrifice all the oneshot commands, they will be reintroduced as
client-calls to the daemon.
