Column by column parallel subsetting. Closes #1660. Closes #1883.
Column by column parallel subsetting. Closes #1660. Closes #1883.

jangorecki/data.table,jangorecki/data.table,Rdatatable/data.table,Rdatatable/data.table,vlulla/data.table,Rdatatable/data.table,jangorecki/data.table,vlulla/data.table,jangorecki/data.table,vlulla/data.table,vlulla/data.table,Rdatatable/data.table