Adding timeout to alert.post and http_post nodes.  Currently
http.DefaultClient is used, which blocks indefinitely when the endpoint
accepts the connection but does not respond to the HTTP request.
Switching to using a non-default http.Client with a timeout defined per
node.

bufpool provided buffers result in race condition when used with a
buffer user that expects to take ownership of the buffer (like http
client appears to).  Returning an extended version of Buffer that allows
it to be closed, where the closing the buffer performs a Reset() on the
buffer and returns it to the pool it came from.