Fix choppy TPS graph.

The server may return cached stats which is the same as before. Using the old
cached values will cause the TPS to drop to 0. Skip cached results if the
timestamp is the same as last time.
Fix throughput calculation.

DrawTPSChart() was not detecting cached stats results from the server. When a
cached result is returned, the calculated tps would be 0. Ignore cached
results.

Switch to using the timestamps in the stats results to calculate the duration
and the throughput, so that the function doesn't depend on the refresh frequency
to be 1 second. Even if it's 1 second, due to network delays and scheduling, it
could be more or less than 1 second.
Bound throughput chart memory usage.

Only keep the last minute's throughput numbers in the chart.
Fix time axis in the throughput chart.

It takes milliseconds as timestamps, not seconds.
Show times under the charts in local time.

Limit the number of ticks in the charts so that they don't overlap each other.
Prepopulate the throughput chart so that it doesn't compress at the beginning.
Merge pull request #5 from VoltDB/choppy-tps

Chart improvements