Added ability to specify disk buffer size in TCPOutput plugin. Fixes #1110

@rafrombrc Hi, I'm creating this PR to easier work/progress on this issue. Could you do initial review? I'll create proper tests and documentation in next commit log, but i would really appreciate your initial thoughts or suggestions.
Thanks in advance. 
Oh your right, i see. Yeah that should be fine.

In general I was thinking since we're keeping track of the queueSize on writes, we might even be able to avoid this except on plugin startup. But in general, this seems fine.