box: rename result_process to result_process_perform

We're going to use the 'result_process' name for a function that
processes a result without creating a result_processor struct.

Follow-up commit 21e2def9 ("box: introduce result processor").

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit 3698d270)