support multiline parser in in_tcp plugin

[jpraet]

From the doc:

Unlike other parser plugins, this plugin needs special code in input plugin, e.g. handle format_firstline. So currently, in_tail plugin works with multiline but other input plugins don't work with multiline.

It would be nice to support multiline parsing in the in_tcp plugin as well. I am using in_tcp to process log events that are delimited by '\n'. But the log message inside the log event might also contain '\n'. So with the multiline parser I would be able to aggregate those in a single log event, which is currently not possible.

[kenhys]

v0.12 is not maintained anymore, I'll close it.