tools: add mixed read-write performance evaluation scripts

[wilsonwang371]

This is the code change mentioned in issue #13001

It consists of three parts:

1. New benchmark command "txn-mixed". This will run a mixed load of read(txn range request) and write(txn put request) operations.
2. rw-benchmark.sh script. This will run txn-mixed benchmark with a range of different parameters and save the output to a csv file. This script will bring up only one etcd server.
3. Python script plot_data.py. This python script will plot the data and generate a graph. If with two input csv data files, it will also generate a column of the delta between two csv data files.

Command: ~/etcd-master/tools/rw-heatmaps/rw-benchmark.sh -w /data00/etcd-tests/
result.csv

./plot_data.py ~/result.csv -t "Main Branch Mixed R/W Performance" -o single_result.jpg

./plot_data.py ~/result.csv ~/result.csv -t "Main Branch Mixed R/W vs. Itself Performance" -o two_results.jpg

@ptabor @gyuho

[wilsonwang371]

Just did a small change to update the delta color map

[hexfusion]

This is great thank you!