feat(benchmarking) : Producer-Consumer scenario

This patch adds the ability to benchmark Marconi. The benchmark tool
is a console script, and can be triggered using

$ marconi-bench-pc

The Benchmark tool fires up both a Producer Process and a Consumer
Process, while accepting CLI parameters for the number of processes,
number of workers and duration of test.

The Producer Process publishes messages to a given queue, while the
Consumer consumes the messages by claiming and deleting them.

Setup:

Benchmark dependencies need to be pip installed:

 pip install -r bench-requirements.txt

Export an environment variable called MESSAGES_PATH and set it to the
path of messages.json in marconi/bench

Note: This allows benchmarking with different set of messages rather
than those specified in messages.json

Usage:

$ marconi-bench-pc -p {No. Processes} -w {No. Workers} -t {No. Seconds}

Example:

$ marconi-bench-pc -p 2 -w 2 -t 4

Partially Implements: blueprint basic-benchmarking
Change-Id: I57ebe853554199490adba8b2a091423f399b0565
Merge "feat(benchmarking) : Producer-Consumer scenario"