Drain collector buffer on Close

[richardmarshall]

Update the Close() logic of the HTTP and Scribe Collectors to drain any
pending spans before returning. This provides a mechanism to prevent
loss of buffered trace data when shutting down.

This is of particular use for short lived processes such as cli clients.

[basvanbeek]

Thanks for the contribution Richard. I will check the code on Friday when done with meetings and traveling. I quickly glanced at it and it looks good but want to give it a tiny bit more attention.

If it works properly I will also update the CLI examples as they currently use a wait for the buffer to be flushed.

[richardmarshall]

Thanks for the update. I can quickly add the CLI example change to this PR. Didn't notice it when I was updating the tests to make use of the new Close logic.

[basvanbeek]

Thanks for the contribution Richard