Exporter stack executor

Adds a stack for exporters that get executed at the end of the request process. Until now, during a request, if two export operations occurred (e.g.- due to two calls to save()) two unique ids are generated and two build request are called. Now, with the stack executor, only one unique id is generated.

Using the stack executor is optional, so Drush commands and custom exporters work as usual, without breaking changes.