prepare 5.12.0 release

[LaunchDarklyCI]

[5.12.0] - 2020-04-03

Added:

• New configuration option streamInitialReconnectDelayMillis allows setting the initial delay (not including jitter) for stream reconnections.

Changed:

The stream reconnection behavior has been changed to be consistent with other LaunchDarkly SDKs, as follows:

• Previously, there was a fixed delay of 125 milliseconds for the first attempt to reconnect after a stream failure, which would then increase with an exponential backoff for subsbequent retries up to a maximum of 15 seconds (but the backoff was used only after HTTP error responses, not connection failures).
• Now, the delay starts at 1000 milliseconds, unless changed with streamInitialReconnectDelayMillis.
• The new maximum is 30 seconds.
• The backoff now applies to all types of stream failures.
• There is now a jitter that randomly reduces each delay by up to 50%.
• The delay now resets to the initial level if the stream has been active for at least 60 seconds.