feat(util): add worker queue

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): process theorems in parallel

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(util/worker_queue): bug in join method

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(util/worker_queue): bug in join method