Add read queue to workspace mutex

[msujew]

Right now, the workspace mutex is only able to lock on write operations - any read operation will abort the current write operation. We want to extend the mutex to allow to perform a read operation (or multiple ones in parallel) after finishing all queued write operations.

This could also resolve the issue experienced in #1290, since it's probably related to an LSP service trying to resolve a cross reference during the build process.

[spoenemann]

We already had a read-write lock in a previous PR: 56966f6#diff-095a6181fde780365fcff35aa085c4b5aff0ec9e081d26f716bd29bca6575974

We decided to reduce it to a simple mutex because we wanted to keep things simple. At that time, I didn't see the case where a language developer adds a custom operation that needs read access at a point in time where the document is in a clean state. We could revive that old code now.