Move Multi* and SlowMultiReaderWrapper to contrib [LUCENE-2859]

[asfimport]

We should move SlowMultiReaderWrapper and all Multi* classes to contrib as it should not be used anymore.

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Migrated from LUCENE-2859 by Uwe Schindler (@uschindler), updated May 09 2016

[asfimport]

Uwe Schindler (@uschindler) (migrated from JIRA)

One use case is still there: Merging of segments needs Multi* so we cannot remove them from core. But we should hide as it's not public API.

In all cases, SlowMultiReaderWrapper should be moved to contrib.

[asfimport]

Jason Rutherglen (migrated from JIRA)

Merging of segments needs Multi*

Why's this?

[asfimport]

Michael McCandless (@mikemccand) (migrated from JIRA)

We could fix that merging uses MultiDocs/AndPositionsEnum. It's not particularly clean because we make Mapping* subclasses to remap the docIDs around deletions. If, instead, we fixed PostingsConsumer.merge to take the subs' enums, instead of a single multi enum, then that method could go segment by segment.

[asfimport]

Steven Rowe (@sarowe) (migrated from JIRA)

Bulk move 4.4 issues to 4.5 and 5.0

[asfimport]

Uwe Schindler (@uschindler) (migrated from JIRA)

Move issue to Lucene 4.9.