keep-alive component shared cache
What problem does this feature solve?
with the shard cache, we could pop the page to a window like component (or vise versa) without loose any infomation. otherwise one KeepAlive component only keeps its own vnodes
What does the proposed API look like?
just add a prop indicates KeepAlive component uses shared/local cache
Duplicate of https://github.com/vuejs/core/pull/4339 and the linked RFC