toolhead: Rework lookahead flush to be more stable during high cpu

Change the lookahead queue so that it attempts to buffer at least
buffer_time_high amount of moves when first starting a print.  This
helps ensure the buffer is normally always full.

If the buffer falls below buffer_time_low then it is either due to the
end of a print or because octoprint/klippy is unable to keep up.
Change the code so that in this case the lookahead queue will attempt
to gather buffer_time_high amount of moves before restarting movement.

Update the default buffer_time_low to 1 second and buffer_time_high to
2 seconds.  With the above changes a smaller buffer_time_high and a
larger buffer_time_low are more practical.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/mcu.py

@@ -393,7 +393,7 @@ class MCU:
             def dummy_set_print_start_time(eventtime):
                 pass
             def dummy_get_print_buffer_time(eventtime, last_move_end):
-                return 0.250
+                return 1.250
             self.set_print_start_time = dummy_set_print_start_time
             self.get_print_buffer_time = dummy_get_print_buffer_time
     def timeout_handler(self, eventtime):