Add udp and tcp input

[repeatedly]

General approach for resolving #363

@kiyoto Could you check this PR?

[kiyoto]

I will take a closer look and test today. It might be a good idea to check with @frsyuki why the standard socket library wasn't used initially.

[kiyoto]

@repeatedly

I tested this locally on Ubuntu Precise VM.

• Checked that it can handle both UDP and TCP payloads with csv/regex.
• I noticed that for TCP + format none, the "\n" at the end is preserved. I suppose this is an expected behavior?
• Most importantly, I checked that this works with syslog! Hooray! I will write a new solution page once this patch lands.

[repeatedly]

@kiyoto Thank you for testing.

I noticed that for TCP + format none, the "\n" at the end is preserved

Hm. I can remove @delimiter from message easily.
Does in_tail removes \n? If so, keep same behaviour is better.

[repeatedly]

I also add "source_host_key" option to add source host to records.
I'm not sure this is useful or not in general.
If users use in_udp or in_tcp for various sources, it might be useful for detecting log source.

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@frsyuki @kiyoto @tagomoris @sonots Do you have any concern to merge this PR?

[tagomoris]

I think that using TcpInput directly is obsolete now.
If it is correct, we should add warn logs for direct use (maybe both of tcp/udp).

[repeatedly]

No. Old TcpInput is obsoleted since 2 and half years ago so there is no users.
This PR introduces new TcpInput to handle general TCP access instead of old obsoleted TcpInput.

[tagomoris]

I see.

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@kiyoto Follow in_tail way. Now in_udp and in_tcp removes a delimiter from message. Please check.

[repeatedly]

If there is no problem, I will merge this PR tomorrow today.