Main goals are:
1. Ability to use mocks in unit tests instead of
having to setup mock web server as if it was an
integration test.
2. Cache Retrofit services in memory
3. Make it easier to read
4. Use OptIn where possible instead of propagating
Experimental* annotations everywhere
It seems that it is possible to launch several coroutines on same
main thread of application. That's why it is possible to launch both
invoke and invalidate at the same time even though they are marked as
synchronized. To fix the issue this commit uses a concurrent collection
instead of synchronization.
