Material Tabs with lazy loading rendering duplicate content

#### Bug, feature request, or proposal:
Material tabs (with `matTabContent`) are rendering multiple instances of the tab body inside the same tab when switching away and back too fast.

#### What is the expected behavior?
Not seeing duplicate content.

#### What is the current behavior?
There are multiple instances of the same body/content inside.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-material2-issue-jwxe85
Switch back and forth quickly on these tabs and pay attention to what happens to the one in the middle. You will notice content being rendered multiple times.

#### What is the use-case or motivation for changing an existing behavior?

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?

#### Is there anything else we should know?

从链接的 StackBlitz 复现开始，使用 matTabContent 快速切换 Material 标签页，以观察重复的内容。跟踪标签页切换涉及的 lazy-loading 行为，然后验证快速切换离开并返回后，每个标签页的内容只留下一个实例。

由索引模型根据 Issue 内容生成。