Task system: Send incoming tasks directly to waiting executors

[eddyashton]

This PR contains some follow-up cleanups from the discussion in #7269:

• Returning const std::string& from the get_name calls to avoid any additional copies. The name is generally per-type, can opt in to per-instance where appropriate, in either case it should be formatted/constructed eagerly.
• Removing the .empty() method. Replaced by a get_summary() returning some basic stats about the current queue state.
• API simplification, removing unnecessary IJobBoard and clarifying which tests (most) are testing a local JobBoard vs the global static ccf::tasks.

Also a perf improvement - rather than gluing together discrete condition_variable and locked queue (taking 2 separate mutexes in sequence) we use a condition_variable wait for workers to pause directly on the work queue.