Save 64 bytes RAM per thermistor by moving median_buffer to AHB0.

Also we only need one median_buffer (total), because each thermistor
uses it only when quick_median() is running. Actual data is stored
in the queue. So this patch reuses the same buffer for all thermistors.
Merge pull request #567 from PetteriAimonen/save_thermistor_ram

Save 64 bytes RAM per thermistor by moving median_buffer to AHB0.