Summary:
This is a diff for managed iterator. A managed iterator
is a wrapper around an iterator which saves the options for that
iterator as well as the current key/value so that the underlying iterator
and its associated memory can be released when it is aged out
automatically or on the request of the user. Will provide the automatic release as a follow-up diff.

Test Plan: Managed* tests in db_test and XF tests for managed iterator

Reviewers: igor, yhchiang, anthony, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D31401