refactor: persist workspaces across connections

[arendjr]

Summary

Changes the daemon logic to reuse workspaces across connections.

Motivations:

• This could be a significant break in expectations for some users, even though I expect most to be unaffected. But this means that if we want to change the logic, we better do it before the 2.0 release.
• I'm preparing to add a filesystem watcher for the workspaces, and if every connection gets its own workspace, that means every connection needs to get its own watcher too. That seems undesirable, because duplicate watchers wouldn't just do duplicate work, they would also increase the open file count, which can be problematic especially on Linux.
• The code seemed to suggest that not sharing workspaces was an oversight, so given the above, why not fix it now?
• If we don't do this, why do we allow the CLI to opt into using the daemon at all? Connecting to the daemon from the CLI only makes sense IMO if you get access to shared caches... it doesn't make much sense if you get a fresh workspace.

Note: This only applies to the daemon, so it mainly affects the LSP server which uses it. The above applies when multiple connections from clients are established to the same daemon. CLI sessions don't use the daemon by default and are unaffected unless you explicitly opt-in.

Test Plan

Added test case.

[ematipico]

Thank you; this is a great fix! I would add a changeset because you mentioned that this could be a breakage for some people.