pdudaemon and merge policy for network PDUs

[Bastian-Krause]

pdudaemon already supports a wide range of network PDUs. labgrid supports interaction with pduaemon (PDUDaemonPort, PDUDaemonDriver).

We don't want to duplicate functionality. Issues like Gude's IP blocking (#586) will not happen because all requests to a PDU come from the same daemon.

• Add support for synchronous pdudaemon behavior to PDUDaemonDriver, see use asyncio/aiohttp and drop database/queueing pdudaemon/pdudaemon#78 and PDUDaemonDriver improvements #1154
• add a way to pdudaemon to retrieve the port's status: remember the latest state set in the common driver, which could be overwritten in each driver individually by an actual status retrieval from the PDU
• implement PDUs used in @pengutronix' lab in pdudaemon, set up pdudaemon for our network PDUs, test/use it ourselves
• we'll still merge network PDU (power) drivers for the time being, but we try to mention pdudaemon as an alternative
  • when pdudaemon is well-tested, supports port status retrieval and does releases regularly, we might change this policy

See also #109, #161, #464, #595, #606, #635, #638, #1154