Currently, there is no limit to incoming line length, so an evil client
could cause a memory exhaustion DoS by issuing very long lines.

This patch fixes the bug by limiting the size of the lines.

To do that, we replace the textproto.Conn with a pair of buffered reader
and writer, which simplify the code and allow for better and cleaner
control.

Thanks to Max Mazurov (fox.cpp@disroot.org) for finding and reporting
this issue.