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. 
Needs slight tweaks, and should include the units, the default, and explain what 0 means:

"Defines maximum queue buffer size, in bytes. Defaults to 0, which means no max."