Add max latency and min throughput settings. #926

@keithbostic - I whipped this up (fair warning - pre-caffeinated...).  See if it is in the ballpark of what you had in mind.  A few things:
1.  This requires sample_interval be set to be effective.  But I don't explicitly check and complain if the user doesn't do that.  It just does nothing.
2.  max_latency units are seconds.  That might be too large and we might want usecs or msecs.  I chose seconds because all other current wtperf time units are seconds.
3.  I changed the internal meaning of `cur_reads` etc to be per-second rather than per-interval.  I suspect people will look at the `monitor` output file and set min_throughput to some value similar to those.  The only use of the cur_ variables was to display them per-second anyway.