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. 
If `Wait()` returns an error it means that we've exhausted the specified number of retries, as mentioned above. Currently if that happens we'll drop messages on the floor.