kiji-modeling: EXP-304. Rename AvroColumnRequest -> AvroColumnSpec

issue: https://jira.kiji.org/browse/EXP-304
review: https://review.kiji.org/r/1372
commit: https://github.com/kijiproject/kiji-modeling/commit/b34f90b0ce14531f5ad98d0f92e682da3284518a
kiji-express-music: EXP-304. Rename AvroColumnRequest -> AvroColumnSpec

issue: https://jira.kiji.org/browse/EXP-304
review: https://review.kiji.org/r/1373
commit: https://github.com/kijiproject/kiji-express-music/commit/6ca9bdca67dc3d56acd03afc487a202a6fdd0576