Add rate limiting to creation of system.profile entries

The profiler can be used to capture information about slow queries against a particular database. Currently, the threshold in milliseconds over which a query is considered slow, as well as a probabilistic sample rate, can be used to configure how much data is stored in the system.profile collection. This is important because collecting the diagnostic info has its own storage requirements and can impact performance.

Under periods of heavy load or poor performance, however, the profiler can begin to generate entries at a high rate, which has a further negative affect on performance. In order to avoid this feedback loop, users may which to configure a maximum rate at which slow queries are collected. For instance, users might wish to record at most one slow query every minute, or at most 10 slow queries every second.