Implement mode where one clangd runs per session

This is likely required for people with a workflow where many projects (especially with cross dependencies) are in a session, and are not usually opened in isolation. The necessary steps are:

• Let user choose between "project mode" and "session mode" in settings.
• In session mode, let the "fallback client" serve everything. Generate a merged compile_commands.json (or use settings.compilationDatabaseChanges; investigate how the index location is constructed then).
• On the LanguageClient side, tweak the assumptions about document/project and project/client relationships accordingly.