switch to a priority queue for pending tasks

[ghost]

Some features potentially make a lot of calls to merlin in rapid succession and/or take a long time (relatively speaking) to complete. Examples of this are the codelens feature and diagnostic refreshing for the entire workspace (currently disabled). Find all references for workspace is another one since it will need to do searching and reverse lookups.

In order to keep the more interactive features like hover, outline, and code splitting responsive while these other features are executing, we should really be using a priority queue instead of just running everything in a straight sequence.