ScopedDebugInfoStack::getAll() returns std::vector<std::string> by calling toString() for each scoped debug info in the stack.

However, this function is mainly used in signal handler, which is problematic, because we might not be able to allocate memory when handling a signal.

We should change signature of ScopedDebugInfoStack::getAll() and ScopedDebugInfoStack::Rec::toString(), so that they can return information without forcing an allocation.

For example, we can use appendTo semantics and pass an ostream or bson obj builder  variant that doesn't do heap allocations.