fix(send): Allow _serializeBuffer() to be async

lib/logger.js: use Promise.resolve().then() to call _serializeBuffer, so it can optionally be async in subclasses.
test/logger-log.js:
test/logger-errors.js: Add tests for async _serializeBuffer, and error handling thereof.

Note: this looks like a large change but it's not really -- just diff getting confused by the indentation changes The substance of the change is to go from this:

const data = this._serializeBuffer(buffer)
// do stuff with data

to this:

Promise.resolve(this._serializeBuffer(buffer))
  .then((data) => {
    // do stuff with data
  })
  .catch((error) => {
    // Handle errors in resolution of _serializeBuffer
  })

I did not try to refactor the code in // do stuff with data because it's got a bit too much dependency on shared state vis a vis local variables. I think it's possible to refactor it to make that a bit cleaner but I think that's out of scope for this change.

logdna / logger-node

fix(send): Allow _serializeBuffer() to be async #71