IndexWriter.deleteDocuments should have option to not apply to docs indexed in the current IW session [LUCENE-2679]

In #3729 we are struggling with how to handle buffered deletes,
with the new per-thread RAM buffers (DWPT).

But, the only reason why we must maintain a map of del term -> current
docID (or sequence ID) is to correctly handle the interleaved adds &
deletes case.

However, I suspect that for many apps that interleaving never happens.
Ie, most apps delete only docs from **before** the last commit or NRT
reopen. For such apps, we don't need a Map... we just need a Set of
all del terms to apply to past segments but not to the currently
buffered docs.

And, importantly, with #3729, this would be a single Set, not
one per DWPT. It should be a a healthy RAM reduction on buffered
deletes, and should make the deletes call faster (add to one set instead of
N maps).

We of course must still support the interleaved case, and I think it
should be the default, but I think we should provide the option for
the common-case apps to take advantage of much less RAM usage.

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Migrated from [LUCENE-2679](https://issues.apache.org/jira/browse/LUCENE-2679) by Michael McCandless (@mikemccand), updated May 09 2016