LUCENE-5969: recreate branch for phase 3

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1629503 13f79535-47bb-0310-9956-ffa450edef68
merge trunk

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1631928 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: add 5.10 dv with segment header, CONST optimization, and missingBits ghostbuster

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LUCENE-5969: add tests

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merge trunk

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1632275 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: use sparsebitset to expand sparse encoding to cover more absurd cases

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LUCENE-5969: fix tests

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LUCENE-5969: move old postings back compat to backward-codecs, cleanup PBF related stuff, add segment headers, etc

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LUCENE-5969: correct TODOs

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LUCENE-5969: woops, revert

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1633212 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: correct TODOs

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LUCENE-5969: clean up constants

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1633385 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: Copy block tree to backward codecs for 4.0-4.10, remove conditionals in 50 version, add getStats() TermsEnum API to remove need to expose Stats impl

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LUCENE-5969: move blocktree stats -> stats api

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LUCENE-5969: add missing @Override

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LUCENE-5969: fail precommit on typo'ed TODO

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LUCENE-5969: fix TOODs and add missing license header

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LUCENE-5969: Move bwc blocktree writer to test module, update 50 writer to use new header reading/writing

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1633465 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: test that default codec uses segmentheader for all files. change .si to write its ID the same way for consistency

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LUCENE-5969: segmentHeader -> indexHeader, cut over segments_N, detect mismatched .si, consistency of encoding elsewhere

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