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

postgrest.cabal

@@ -53,7 +53,6 @@ library
                       PostgREST.Error
                       PostgREST.GucHeader
                       PostgREST.Logger
-                      PostgREST.Middleware
                       PostgREST.MediaType
                       PostgREST.Query
                       PostgREST.Query.QueryBuilder