improve performance for update-only case

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In many realtime search applications, there are no deletes.
In the case where recall is high, for each hit candidate, we need to check to 
see if it is in the 
modSet, and can be inefficient.

The idea is at indexing time, the active ram directory before adds a doc into 
the modSet, checks 
against the backup ram reader and disk reader for existence, and add to the 
modsets appropriately.

In the update only case, the modset should be empty and thus avoids the check 
penalty.

Original issue reported on code.google.com by john.w...@gmail.com on 11 Nov 2009 at 6:40

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Original comment by jiong.w...@gmail.com on 13 Nov 2009 at 7:12

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Original comment by john.w...@gmail.com on 15 Dec 2009 at 6:50

• Changed state: Fixed