[ffapi] Ensure Default Log Level is Set in Handler

[onelapahead]

Following up from #201

Its a bit awkward - we added a private field to HandlerFactory. But HandlerFactory doesn't have a proper New func or Init to guarantee that internal log level field is set. If the log level field is not set and defaults to 0, then logrus panics on the first log message.

For most clients, they let APIServer construct and setup a HandlerFactory, so they are unaffected. But if anyone was initializing their own HandlerFactory and doesn't then call SetAPIEntryLoggingLevel, it would panic.

We don't want to check for 0 on every API request, as that would be inefficient, so we check when we construct APIWrapper's who would depend on the log level to not be nil in order to fix this.

In a 2.0 of ff-common, we should likely make all fields of HandlerFactory private, have a New func, and use the functional options or builder pattern to provide a variadic way of setting all the possible fields (which is what a struct with public fields and no constructor feels like, but has potential for bugs like this one).

[EnriqueL8]

Thanks @onelapahead !