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. 
This is good, just slightly awkward wording. I'd rename the option to "queue_full_action", and reword the following text to:

"Specifies how Heka should behave when the queue reaches the specified maximum capacity. There are currently three possible actions:"