Note: Use end callback when dealing with file streams

[jcrugzz]

Note for you @indexzero when reimplementing the file-transport.

When switching to a new file to write to you can:

1. create a new stream that would begin getting written to
2. end the stream and pass it the callback to know when its flushed everything in its buffer to disk.

Random notes:

• The drain event is called when the amount of data in the buffer reaches the highWaterMark
• What this means is that logs are not flushed to disk when the drain event is called but it means we can begin writing to it again from a backpressure standpoint (if we ever hit that threshold when writing to a file).
• The only way we know when logs are flushed to disk is when the the callback passed to end is called or the finish event is called if end was called without passing a callback.
• The above may not hold true during uncaughtExceptions but i think it will get us pretty close.

[indexzero]

Closing as a duplicate of #1144 – which is now resolved \o/