fix(emacs/lean-company): send dummy continuation when send wait
chore(emacs/lean-server): send signal info to lean-server-trace
feat(lean-server): handle signals (error/quit) in continuations

Related with #158
feat(emacs/lean-server): async task queue holds type of cmds as well
refactor(frontends/lean): replace collect_metavars with metavar_closure helper class