MINIFICPP-1313 Do a multifile lookup in onSchedule

[fgerlits]

https://issues.apache.org/jira/browse/MINIFICPP-1313

Do a first multifile lookup in onSchedule() to make sure TailFile can see the files which were already present before minifi started. This fixes an intermittent CI failure caused by TailFileTests running on the virtual machine less than 10 minutes after bootup.

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[arpadboda]

The change itself looks good, my comment might end in a follow-up.