MB-100: log "took too long" memcached messages as debug

I.e. because it's still too common and is largely diagnostics for us
and not really error.

Change-Id: Iabb8b7bba960e3164ac8c49457d6b374c402ccaa
Reviewed-on: http://review.couchbase.org/30219
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>