fix: resource embedding opens empty transaction

This required untangling logic from App.hs.
Building/rejecting a plan no longer consumes a pool connection.

* Added io test for failed resource embedding not consuming connection
* optionalrollback to Query.hs and Response.hs
* delete Middleware module
* remove plan logic from App.hs
* remove RequestContext
* remove pkCols logic from App.hs
* remove proc logic from App.hs
* remove config logic from handleRequest

src/PostgREST/Plan/MutatePlan.hs

@@ -22,6 +22,7 @@ data MutatePlan
       , onConflict :: Maybe (PreferResolution, [FieldName])
       , where_     :: [LogicTree]
       , returning  :: [FieldName]
+      , insPkCols  :: [FieldName]
       }
   | Update
       { in_       :: QualifiedIdentifier