Arexx Data Logger Protocol
~~~~~~~~~~~~~~~~~~~~~~~~~~
All packets have fixed size of 64 bytes. The first byte is a packet
type, followed by type-dependent data.

Timestamps are 32-bit integers representing the number of seconds
since 2000-01-01 00:00:00 UTC.

Request packets  (from the PC to the logger)
~~~~~~~~~~~~~~~

type 03		Request sensor data
		The data part is probably unused.

type 04		Set clock
		Data:	u32le	timestamp


Reply packets  (from the logger to the PC)
~~~~~~~~~~~~~

type 00		Report sensor data
		The data part is a sequence of tuples, each tuple
		starts with its length.

		9-byte tuples:
			u8	tuple length (0x09)
			u16le	sensor ID
			u16be!	raw value
			u32le	timestamp

		10-byte tuples contain one extra byte:
			u8	signal quality (units unknown)

		12-byte tuples for new sensors with long IDs:
			u8	tuple length (0x0c)
			u32le	sensor ID
			u16be!	raw value
			u32le	timestamp

		A 0-byte tuple serves as a terminator.