Make latency histogram collection optional in full-time diagnostic data capture

The new latency histogram facility adds a considerable amount of data to ftdc, decreasing the retention period significantly. It has limited value for the primary purpose of always-on ftdc, because mongod logs provide information about the high-latency operations which are generally of interest for after-the-fact issue analysis. Collection of latency histogram data by ftdc should be optional, off by default. It should be possible to enable capture of latency histogram data in ftdc dynamically at runtime if desired to use it for targeted performance investigations.