Hot Standby btree delete records and vacuum_defer_cleanup_age

Heikki pointed out to me that the btree delete record processing does
not respect vacuum_defer_cleanup_age. It should.

Attached patch to implement that.

Looking to commit in next few hours barring objections/suggestions, to
both HEAD and 9_0_STABLE, in time for next minor release.

--
 Simon Riggs           http://www.2ndQuadrant.com/books/
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