[Better Engineering] Improve CI build time

[rybalkinsd]

Live is too short.
Current build time is 5:30 avg.
Check latest builds: https://travis-ci.org/rybalkinsd/kohttp/builds

Target is to cut it to 2 min or 1 min ideally.

How to do this:
We have a lot of tests at the moment with a good coverage.
However not all of our tests are good:

• it is possible to move some tests from integration to unit with mocks

• it is possible to simplify tests with sleeps, like RetryInterceptorTest

• investigate codecov problem on CI example

  X> Failed to upload
  -> Sleeping for 30s and trying again...