node-gdal: Prototype async function with error reporting

My sense is that async functions can wait: once sync bindings are in place with solid tests and some releases are out in the wild then selectively exposing some critical functions as async would be worthwhile.

But we should figure out now how we think async functions will work. In particular error reporting will be important to discuss. This came up in the comment stream in #13.

The questions:

• Can CPLSetErrorHandler be set, used, and unset safely per async function: so is is safe to use while in the libuv threadpool?
• Can CPLSetThreadLocalConfigOption be used similarly to allow GDAL config options to be modified per async function?
• If the answers to the above issues are no, then what options do we have for catching and reporting meaningful errors when async functions hit GDAL errors - Is this a case where the C++ API instead of the C API should be used?