Fix few minor problems in nodeprobe cfstats

nodeprobe cfstats reports that readlatency/writelatency is NaN on the keyspace level although it obviously is not.

For example:
Keyspace: Keyspace1
Read Count: 392
Read Latency: NaN ms.
Write Count: 262
Write Latency: NaN ms.
Pending Tasks: 0

Column Family: MyCF
Memtable Columns Count: 143
Memtable Data Size: 123433
Memtable Switch Count: 2
Read Count: 392
Read Latency: 0.533 ms.
Write Count: 262
Write Latency: 0.000 ms.
Pending Tasks: 0

Column Family: Standard2
Memtable Columns Count: 0
Memtable Data Size: 0
Memtable Switch Count: 0
Read Count: 0
Read Latency: NaN ms.
Write Count: 0
Write Latency: NaN ms.
Pending Tasks: 0

The problem here is that there is more than one cf, and one of them has read latency/writelatency NaN. This causes the keyspace readlatency/writelatency to be NaN instead of the average across all cfs.

Another problem with cfstats is that it does not account for the delays when a read/write times out, so it does not accurately reflect the health of the system under too much stress.