Complete ETag piping through the query path

Reviewed-by: marc.millstone@socrata.com
Plumb "If-Modified-Since" headers through for exports and queries

Reviewed-by: marc.millstone@socrata.com
Plumb error through for creation of row ID on a non-unique column

Reviewed-by: marc.millstone@socrata.com
Implemented getColumn (lookup column metadata)

Reviewed-by: marc.millstone@socrata.com
Added liquibase migrations

Reviewed-by: marc.millstone@socrata.com