LUCENE-5969: recreate branch

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LUCENE-5969: assert -> check

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LUCENE-5969: assert -> check

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LUCENE-5969: copy over cruft for back compat

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LUCENE-5969: remove some cruft, header -> segmentHeader

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LUCENE-5969: cutover all merge implementations away from LeafReader[] to individual low-level producers

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LUCENE-5969: fix compile/javadocs, tighten up backwards codecs, add more safety to 5.x fields/vectors

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LUCENE-5969: add merge api

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LUCENE-5969: add missing checkIntegrity() calls for segments that cannot be bulk-merged

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LUCENE-5969: current state for dv/norms

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LUCENE-5969: add segment suffix safety

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LUCENE-5969: fix nocommit

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LUCENE-5969: clean up unnecessary back compat and add segment header

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1628679 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: clean up unnecessary back compat and add segment header

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LUCENE-5969: clean up unnecessary back compat and add segment header

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LUCENE-5969: more cleanups

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LUCENE-5969: give remaining codecs segment headers and cleanups

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LUCENE-5969: fix formats

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LUCENE-5969: start improving CFSDir

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LUCENE-5969: move CFS to codec

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