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. 
`SetPipelineConfig` only needs to be implemented if you need access to the pConfig *before* the Run method is called. Once the Run method is called you have the PluginHelper, and you can just call PluginHelper.PipelineConfig() to get what you need.