Werk #1671: windows agent: fixed gaps in eventlog monitoring after agent restart

The windows eventlog monitoring was not entirely continuous. After the agent started it determined determined the initial record number for each eventlog. Per default the start number was the last record. Thus resulted in monitoring gaps as long as the windows was not running, e.g. windows startup.

With this fix, the windows agent now uses a statefile (eventstate.txt) which contains the last processed eventrecord ids for each eventlog. On startup the statefile is read and the record numbers are set accordingly.

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