Port NUTCH-1475 Index-More Plugin -- A better fall back value for date field to 2.x

Continuing directly from NUTCH-1475

the fetch datum (and not the current CrawlDatum from CrawlDb) is passed to IndexingFilter plugins, cf. conversation @user [1] (thanks, liaoks!).
Since fetch datum contains the time the fetching has taken place, we should take this as last fallback value (and not the current time). To use the lastModified time from CrawlDatum (if set) is not wrong and is closer to 2.x

We should address this final part in this issue.