Improve lager_console_backend to support non-atom output log levels.

This supports forms such as "!notice" for configuring the output
log level (using the config handling changes from 7aa316902e2d).
Switch to notify, not sync_notify.

Despite the commit message on d19d927da6, I can't see how sync_notify
could possibly be faster, and I'm seeing performance issues when logging.

Now that we are logging asynchronously, we also need to add flushes in
critical places in the tests.
Merge branch 'async-notify' into tipbit-master