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. 
Hrm. There's some ambiguity here. If the user sets a `max_retries` value, then we'll need to decide what to do if the retries are exhausted, btn either shutting down or dropping messages. I can see two possible solutions:

1. Leave the `queue_capacity_reached` (or whatever we call it) options as they are and *don't* let the user set the retry options, so we can force it to always keep retrying forever.

2. Remove the "block" option, and instead use the presence of a retries section to specify whether or not any blocking will happen. If there's no max_retries, it will block forever. If max_retries is set, then if it gets hit we either drop or exit depending on what was specified for queue_capacity_reached.

I'm leaning towards #1 since it's a simpler solution, and easier to explain to the user.