Change the default sample rate in wtperf to 50 -- sampling every operation introduced some big regressions on the EC2 box.
Cleanup in wtperf including:

* Stop using asserts, handle errors correctly
* Switch all gettime calls to use wt_epoch
* Only track latencies if sample_interval is configured
Fix compiler warning in wtperf:

../../../bench/wtperf/wtperf.c:789:27: error: conversion to 'long long
unsigned int' from 'long int' may change the sign of the result
[-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c:789:13: error: conversion to 'long long
unsigned int' from 'time_t' may change the sign of the result
[-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c:790:30: error: conversion to 'long long
unsigned int' from 'long int' may change the sign of the result
[-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c:790:15: error: conversion to 'long long
unsigned int' from 'time_t' may change the sign of the result
[-Werror=sign-conversion]