fix(library/mt_task_queue): work around non-recursive lock
fix(library/mt_task_queue): work around non-recursive lock

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