Let the client manage the HTTP client

We are currently creating a new OkHttp client in various place, while it is not the right thing to do. OkHttp is maintaining background threads for connection pooling, so this is better to have a single instance, used everywhere.
Since we may also use the IDE provided HTTP client (for example in SLI), we should rework all the APIs so that the client code of SLCORE is responsible to manage the HTTP Client.