Add the "backend_overridable" tag to the server tasks service

See this article by Larry Garfield. While he probably/hopefully exaggerates when he writes that "if you ever write an SQL query yourself in Drupal 8, you're probably doing it wrong", the backend_overridable tag seems like something that could indeed make our server tasks service more flexible. (I'd also be open to use something other than a custom SQL table for the storage, like he suggests (see also here), but I don't think any of the other options is really a good fit.)