read_options.background_purge_on_iterator_cleanup to cover forward iterator and log file closing too.

Summary: With read_options.background_purge_on_iterator_cleanup=true, File deletion and closing can still happen in forward iterator, or WAL file closing. Cover those cases too.

Test Plan: I am adding unit tests.

Reviewers: andrewkr, IslamAbdelRahman, yiwu

Reviewed By: yiwu

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61503