Dense/sparse sparse-chunk constructors

Move from a sparse NewChunk to a Sparse Chunk (either CX0Chunk or
CX2Chunk) without ever blowing out the size.
Add tests of the same.
Handle broken last line on a sparse column
Fix tests that peek into the guts of NewChunk

Fix sparse/dense bitvector path
Merge branch 'master' of github.com:0xdata/h2o