Add option for overriding root spec (#1317)

* Only for pg >= 9.6

* Disallow specifying schema on root-spec

* Increase memory test upper bound

src/PostgREST/Types.hs

@@ -179,6 +179,10 @@ data OrderTerm = OrderTerm {
 , otNullOrder :: Maybe OrderNulls
 } deriving (Show, Eq)
 
+{-|
+  Represents a pg identifier with a prepended schema name "schema.table"
+  When qiSchema is "", the schema is defined by the pg search_path
+-}
 data QualifiedIdentifier = QualifiedIdentifier {
   qiSchema :: Schema
 , qiName   :: TableName