Avoid touching all pages in extent_recycle for debug build.
We may have a large number of pages with *zero set (since they are populated on
demand). Only check the first page to avoid paging in all of them.
This makes it easy to compare performance with and without 'retain:true'.
Discussed with: jasone
Obtained from: Qi Wang <interwq at gwu dot edu>
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
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