[#46, #90] Upgrade Okhttp library to 3.6.0 and enable AOT

[jonathannaguin]

• Upgrade OkHttp to 3.6.0.
• Adding AOT for every project.
• Dropping support for retrofit, new version of retrofit (2.x) has renamed RestTemplate to Retrofit and now its Builder is final. However the builder supports a OkHttpClient as an option, so it might be possible to use comsat-okhttp client as a parameter to the builder.

[jonathannaguin]

@pron could anyone review this PR please? It seems some of the tests are failing but don't think it's related to my changes though

[pron]

This is a very big PR. Thank you for taking the time to do it. But it does do quite a few separate things, and should probably be broken down.

1. Can you explain what you mean by "adding AOT for every project"?

2. Let's separate the removal of Retrofit because a new version is problematic into a separate issue.

[jonathannaguin]

1. In Gradle, I included the compile instrumentation for Fiber in every subproject.
2. Do you want another PR for removing Retrofit? If I rolled back the removal then this PR will not compile at all, as the Retrofit module will stop working at compile time.