MB-100 Grab all xdcr logs in diag_handler.

Change-Id: I4797d24c196e377b14d51bd39f436cf3c78f0b55
Reviewed-on: http://review.couchbase.org/28210
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>