Allow slow op threshold to be customised

During a slow GET investigation for a customer, it was useful to correlate our reporting of slow operations in the memcached log to their application reporting slow requests:

2017-06-28T05:45:34.914126Z WARNING 485: Slow GET operation on connection: 578 ms ([ xxx.xxx.114.151:41590 - xxx.xxx.114.133:11210 ])

However, the threshold in the customer environment differs the memcached threshold - they report ops as slow at 100ms, compared to the fixed threshold of 500ms in memcached. We should allow the threshold to be configured to match with different SLAs.