* Change SSE to use the OkHttp public API only
Previously we prevented end-users from using their own
implementations of Call.Factory because we casted down
to RealCall in RealEventSource.
With this change we're implementing SSE without depending
on any OkHttp implementation details.
This also introduces a new function in EventSources to
create an EventSource.Factory from a Call.Factory, and
hides the previous implementation that required a concrete
OkHttpClient.
Finally this fixes SSE to publish the same EventListener
events as regular HTTP calls.
* apiDump
