cmd: configurable logrus formatters

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.

cmd/config_job_local.go

@@ -90,7 +90,7 @@ func (j *LocalJob) JobStart(ctx context.Context) {
 	// All local datasets will be passed to its Map() function,
 	// but only those for which a mapping exists will actually be pulled.
 	// We can pay this small performance penalty for now.
-	handler := NewHandler(log.WithField("task", "handler"), localPullACL{}, &PrefixSnapshotFilter{j.SnapshotPrefix})
+	handler := NewHandler(log.WithField(logTaskField, "handler"), localPullACL{}, &PrefixSnapshotFilter{j.SnapshotPrefix})
 
 	registerEndpoints(local, handler)
 
@@ -112,7 +112,7 @@ func (j *LocalJob) JobStart(ctx context.Context) {
 	}
 
 	makeCtx := func(parent context.Context, taskName string) (ctx context.Context) {
-		return context.WithValue(parent, contextKeyLog, log.WithField("task", taskName))
+		return context.WithValue(parent, contextKeyLog, log.WithField(logTaskField, taskName))
 	}
 	var snapCtx, plCtx, prCtx, pullCtx context.Context
 	snapCtx = makeCtx(ctx, "autosnap")