[VCR] Add caching support (Replay) for proxy.TestCase

[abhinavsingh]

Is your feature request related to a problem? Please describe.
There can be different scenarios under which proxy.py can be used under test. One such common scenario is to use proxy.py as a proxy server while making HTTP(s) requests to upstream test servers.

Describe the solution you'd like
Add capabilities to:

1. Enable response cache for proxy.TestCase
2. When enabled, proxy.TestCase will cache upstream responses
3. Next time when test is run again with cache enabled, proxy.TestCase will use the cache instead of reconnecting with upstream host.
4. Optionally, write down request / responses in a test fixture file.

Overall, this will speed up test runs.

[abhinavsingh]

This feature is also referred as VCR in developer community. Folks searching for VCR is what prompted this feature request :)