prepare 4.9.0 release

[eli-darkly]

[4.9.0] - 2019-07-23

Added:

• The new Config property LogEvaluationErrors, if set to true, causes the client to output a WARN: log message whenever a flag cannot be evaluated because the flag does not exist, the user key was not specified, or the flag rules are invalid. The error message is the same as the message in the error object returned by the evaluation method. This may be useful in debugging if you are unexpectedly seeing default values instead of real values for a flag. Most of the other LaunchDarkly SDKs already log these messages by default, but since the Go SDK historically did not, this has been made an opt-in feature. It may be changed to be true by default in the next major version.
• The new ldhttp package provides helper functions for setting custom HTTPS transport options, such as adding a root CA.
• The new ldntlm package provides the ability to connect through a proxy server that uses NTLM authentication.

Fixed:

• The SDK was not respecting the standard proxy server environment variable behavior (HTTPS_PROXY) that is normally provided by http.ProxyFromEnvironment. (Thanks, mightyguava!)
• Under conditions where analytics events are being generated at an extremely high rate (for instance, if an application is evaluating a flag repeatedly in a tight loop on many goroutines), a thread could be blocked indefinitely within the Variation methods while waiting for the internal event processing logic to catch up with the backlog. The logic has been changed to drop events if necessary so application code will not be blocked (similar to how the SDK already drops events if the size of the event buffer is exceeded). If that happens, this warning message will be logged once: "Events are being produced faster than they can be processed; some events will be dropped". Under normal conditions this should never happen; this change is meant to avoid a concurrency bottleneck in applications that are already so busy that goroutine starvation is likely.