Audit select! usage for flume channel sends

[rklaehn]

Sending into a flume channel is not cancel-safe. We use select! in our "actors" a lot.

Here is a discussion about flume cancel safety. It seems that while recv_async is cancel-safe, send_async is not, in a subtle way. We should look for places where we send from a select! statement and see if they could be affected.

zesterer/flume#104

[rklaehn]

We are now trying to avoid flume channels wherever possible because of cancel safety issues in both send and recv.