MB-9890: cleanup keys/bodies from xdcr_vbucket_rep crash reports

I.e. so that we cannot leak customer data there

Change-Id: I73d8a8bc263604f6e1f53fa3c38da1c239bca072
Reviewed-on: http://review.couchbase.org/42490
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>