output: Add fast shutdown and make it optional if possible

[jiping-s]

Background

The current shutdown process in fluentd forward client is to attempt waiting for all sent chunks to be acknowledged from server, in order to reduce the numbers of re-sent chunks in the next start.

When there are network troubles, it could take minutes until timeout and during the whole time the input has been shut down, meaning nothing is listening on the incoming port to receive logs from applications. A minute downtime could mean GBs of logs lost, queued up, or blocking applications' operations or triggering even more logs due to logging failure.

What to do

One of the two:

1. Make fast shutdown a global option and support it in fluentd forward client: close everything and assume ongoing buffers lost when the end signal is received - the buffer above output clients should persist all of them for recovery next time.
2. If making it optional is difficult, just replace the old approach completely. The current code may still be needed for output: Limit max duration of outgoing connection to help load balancing #6

Not in Scope

Shutdown handling in other parts should be fine and need no change