Add @CheckReturnValue to detect common mistake through static analysis

A common mistake among new to ParSeq is to treat Task as if it's creation caused is execution e.g.:
```
Task getRelevanceItemsTask = _relevanceBackendClient.get(keys)
.map(relevanceItems -> {
if (Sets.difference(userItems, relevanceItems).size() > 0) {
// Doesn’t actually execute since this is a Task but isn’t connected to parent task
_userItemsClient.update(userId, userItems.addAll(relevanceItems));
}
return _userItems;
});
```
Using jsr305 [@CheckReturnValue](https://static.javadoc.io/com.google.code.findbugs/jsr305/3.0.1/javax/annotation/CheckReturnValue.html) on methods that return Task would allow detecting this mistake through static analysis.

Start by locating methods that return Task and checking the project's dependency and static-analysis configuration for JSR-305 support. Use the example in the issue as the target mistake; done means applicable Task-returning methods are annotated and static analysis flags discarded Tasks.

Written by the indexing model from the issue text.