[ws-manager] improve finalizeWorkspaceContent logic

[sagor999]

Description

This PR does two things:

1. Make sure that we wait for workspace content init to finish before we attempt to dispose it. That way we will not dispose workspace, while another thread is still working on content init of it.
2. Make sure only one finalizeWorkspaceContent is running for a given workspace. This fixes various race conditions, where finalizeWorkspaceContent may get called multiple times (one after another), and each call will attempt to dispose workspace, causing second and any after call to fail, since workspace was already disposed.

Related Issue(s)

Fixes #11710

How to test

Start and stop workspaces.
Start workspaces that fail during init, and ensure this is handled correctly.

Release Notes

none

Documentation

Werft options:

• /werft with-preview

[werft-gitpod-dev-com]

started the job as gitpod-build-pavel-11710.1 because the annotations in the pull request description changed
(with .werft/ from main)

[sagor999]

/werft run

👍 started the job as gitpod-build-pavel-11710.2
(with .werft/ from main)