Terminate after flushing file descriptors

[john-sharratt]

This patch will modify the behavior of a closing WASM process so that it only sends the termination signal after stdout and stderr are flushed.

[syrusakbary]

We need this: Because WCGI will stream the response waiting for either of these conditions:

1. the stdout to finish streaming
2. the process to exit

So if we exit the process before we finish the streaming there is a race condition where the data could be truncated