prepare 2.3.0 release

[LaunchDarklyCI]

[2.3.0] - 2020-05-26

Added:

• EventSource.Builder.logger() and the Logger interface allow a custom logging implementation to be used instead of SLF4J.

Fixed:

• Explicitly closing the stream could cause a misleading log line saying that the connection error handler had shut it down.
• Explicitly closing the stream could also cause an unnecessary backoff delay (with a log line about waiting X amount of time) before the stream task actually shut down.

[eli-darkly]

Actually, let's hold off on this, there is one other change I'd like to include.