FIX tools: latency file output

Make it possible to store latency data in a file for further analysis.

Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
Signed-off-by: Pekka Enberg <penberg@iki.fi>
FIX tools: a script to plot frequency distribution

This is a simple script to plot a histogram given the latency data stored in a
file. You can use it as follows:

$ ./tools/fix/fix_client -n 1000 -o out.csv -m order -p 9000 -h localhost
Client Logon OK
Messages sent: 1000
Round-trip time: min/avg/max = 101.0/188.9/14801.0 μs
Client Logout OK

$ gnuplot -e "file='out.csv'" -e "width=10" -e "max=1000" tools/fix/frequency.gp

Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
[ penberg: Use "round-trip time" for the x-axis label. ]
Signed-off-by: Pekka Enberg <penberg@iki.fi>