* thread-safety and dynamic memory for MessageOutput
* use dynamic memory to allow handling of arbitrary message lenghts.
* keep a message buffer for every task so no task ever mangles the
message of another task.
* every complete line is written to the serial console and moved to
a line buffer for sending them through the websocket.
* the websocket is always fed complete lines.
* make sure to feed only as many lines as possible to the websocket
handler, so that no lines are dropped.
* lock all MessageOutput state against concurrent access.
* MessageOutput: respect HardwareSerial buffer size
the MessageOutput class buffers whole lines of output printed by any
task in order to avoid mangling of text. that means we hand over full
lines to the HardwareSerial instance, which might be too much in one
call to write(buffer, size). we now check the return value of
write(buffer, size) and call the function again with the part of the
message that could not yet be written by HardwareSerial.
