Resolves error message shown.
The following error message is shown when worker_impl.py:199 is run:
```
WARNING:worker(RID,EXPERIMENT):py.warnings:/nix/store/77sw4p03cb7rdayx86agi4yqxh5wq46b-python3.7-artiq-5.7141.1b68906/lib/python3.7/site-packages/artiq/master/worker_impl.py:199: DeprecationWarning: The 'warn' function is deprecated, use 'warning' instead
logging.warn(message)
```
recv() returns 0 instead of data if the socket has already
been closed. This is translated into a zero-length list on
the Python layer. Previously, the code would enter an
infinite loop if the socket was closed while attempting
to receive data.
This partially reverts commit b5e1bd3fa2,
which had removed keepalive. This, however, led to experiments
hanging forever if the core device had dropped the connection
(e.g. to a kernel CPU panic, or the device being rebooted).
The chosen keepalive settings are fairly conservative (with the
10 s timeout) to avoid any possible interaction with smoltcp's
3 s ARP try interval (see GitHub issue #1150), even though this
should be a non-issue now due to the larger ARP cache.
* KC705 master: user can no longer choose whether or not the SMA acts as the 2nd DRTIO channel; SFP and SMA now act as the 1st and 2nd channel respectively by default.
* KC705 satellite: user should now use `--sma` to enable using the SMA as the satellite channel; SFP acts as the satellite channel by default.
* Two DRTIO channels (i.e. satellite and repeater) are enabled by default.
* User can choose either the SFP or SMA as the satellite channel (by passing `--drtio-sat sfp` or --drtio-sat sma` to the argparser), and the unchosen would become the repeater channel.
* One DRTIO master channel is enabled by default.
* User can set the SMA as the 2nd master channel (by passing --drtio-sma to the argparser).
* Multi-channel (i.e. with repeaters) on KC705 satellite is supported but has not been implemented yet.
