MB-8066 Compactor should only evict bodies on request

Change-Id: I476bd3994dc65618af3db478c2fd74381fc4c003
Reviewed-on: http://review.couchbase.org/26504
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>