Improve backend vignette
For speed, don't build vignettes when checking
Replace Query with sql_subquery
Move joins to dbi-s3 and match interface
Update backend vignette
Correct variable name
Make lahman data consistent with nycflights
Add db_save_query
Export sql_escape_*
Make sql_set_op a generic
Consistent use of db_ vs sql_
Rename qry_fields to db_query_fields
db_query_fields and db_query_rows
News item about db backends. Closes #568