now terminals can be initialized with either absolute or relative only,
the other one being chosen sensibly. Also remove race condition when
searching for free addresses.

this way we can read individual object entries without reading
the entire list.

and also improve its output a bit

on some terminals, the PDO can be configured. This
allows for that. It is not perfect yet as we do not properly
overwrite existing PDOs.

uses some internal marked stuff, forgot about the details

now we can keep many terminals operational within the same
EtherCAT roundtrip

until now we only had one asynchronous packet on the wire.
This allows to send them as fast as possible, waiting for them
in bulk.

there is a bug in some versions of Python that asyncio.sleep(0)
cannot be cancelled.

until now this only prints whether the working counter
is wrong. For this to work we had to switch off all
fmmus that somebody else might have already set.

terminals without mailbox only have sync managers for PDOs,
those are then put where normally the mailboxes would be.

this doesn't work yet, the relays do not switch, but maybe the
terminal is broken.

this also means removing some old code and adopting terminals.
Plus, when parsing the PDOs from EEPROM, we guess from the EEPROM
entry number whether this is read or write, not from the entry
proper. At least for EL2624 this is necessary.