AddressSynchronizer.add_address called synchronizer.add, which would only
schedule adding the addr to the Synchronizer in the next event loop iter.
If during that time, the synchronizer called adb.set_up_to_date(True),
the wallet would falsely believe and advertise itself as up_to_date
(as the wallet would see wallet.synchronize not creating new addresses,
and adb (via synchronizer) telling it is up_to_date).
Moments later, the synchronizer._add_address is finally executed and
up_to_date=False propagates out synchronizer->adb->wallet.
Note: this was causing a weird flake8 error on CI. Presumably due to CI running flake8 via py3.7.
```
flake8 . --count --select=$ELECTRUM_LINTERS --show-source --statistics
./electrum/scripts/update_default_servers.py:1:26: E999 SyntaxError: invalid syntax
#!/usr/bin/env python3
^
1 E999 SyntaxError: invalid syntax
1
```
Previously all the interfaces used either "t" or "s".
Now the network only tries to use "s" for all interfaces, except for
the main interface, which the user can manually specify to use "t".
(so e.g. if you run with "--server localhost:50002:t", the main server will use "t",
but all the rest will use "s")
The few other cases that used SimpleConfig.get_instance() now
either get passed a config instance, or they try to get a reference
to something else that has a reference to a config.
(see lnsweep, qt/qrcodewidget, qt/qrtextedit)
