slog-syslog: Streamer3164 doesn't empty buffer after writing

[whipsch]

The Drain::log implementation for Streamer3164 never trims TL_BUF after writing some bytes from it via syslog::Logger::send.

Note that syslog::Logger::send_raw returns the number of bytes written from the RFC3164 formatted message and not the raw message passed to it, so using the count returned by send is not suitable for trimming TL_BUF.

Streamer3164 should probably use format_3164 directly and continuously call send_raw until the message is fully written. Drain::log feels like std::io::Write::write_all to me, so I think this is semantically appropriate.