Allow logger creation during initialization

[elliottt]

Consider only allowing loggers to be allocated during initialization, and throw if Fastly.getLogger is called during request handling.

[JakeChampion]

I believe this isn't meant to be 'only' during initialization, but 'also' during initialization as a way to improve performance but not be a confusing (and breaking) change.

Consider also allowing logger creation during initialization, but then throw if trying to log during initialization. This enables us to get the JS Object into memory during initialization, and during post-initialization when the log methods is called on the Logger instance, we can do the call to the host to retrieve the logger and log to it.

Note: This would mean that the constructor would not throw if called during initialization with a name that is valid but is not a name of an associated Logger on the Fastly Service. That error would be thrown when the log method is called.