fix(library/mt_task_queue): save stack info at thread starting
fix(library/mt_task_queue): save stack info at thread starting

We need this information to get accurate stack information.

Kha/lean,avigad/lean,Kha/lean,leanprover/lean,leanprover/lean,rlewis1988/lean,fpvandoorn/lean,fpvandoorn/lean,johoelzl/lean,johoelzl/lean,digama0/lean,fgdorais/lean,fgdorais/lean,digama0/lean,johoelzl/lean,avigad/lean,fpvandoorn/lean,leodemoura/lean,fpvandoorn/lean,fpvandoorn/lean,leanprover-community/lean,leanprover/lean,johoelzl/lean,fpvandoorn/lean,avigad/lean,leodemoura/lean,dselsam/lean,avigad/lean,fgdorais/lean,Kha/lean,rlewis1988/lean,Kha/lean,leanprover-community/lean,johoelzl/lean,digama0/lean,leanprover-community/lean,Kha/lean,leanprover-community/lean,leanprover/lean,digama0/lean,dselsam/lean,rlewis1988/lean,fgdorais/lean,dselsam/lean,fgdorais/lean,leodemoura/lean,digama0/lean,leanprover/lean,rlewis1988/lean,avigad/lean,leodemoura/lean,fgdorais/lean,leodemoura/lean,dselsam/lean,dselsam/lean,digama0/lean,leanprover-community/lean,leanprover/lean,johoelzl/lean,avigad/lean,Kha/lean,leodemoura/lean,leanprover-community/lean,rlewis1988/lean,rlewis1988/lean,dselsam/lean