[Feature]: add a Close method to App for graceful quit

[yihuang]

Summary

When the app has some asynchronous background tasks running, it might need a hook to cleanup resource gracefully before the node quit.
Specifically for the async commit proposal, we need this hook to wait for the background committer to finish, at least when the quit is a graceful one, like a panic in the upgrade module, so in these cases we don't lose data at all.
Although the async commit feature itself is blocked by current iavl implementation, but similar idea is implemented in memiavl already, so it'd be good to support the cleanup hook first.

Problem Definition

App can do some cleanups when quit gracefully.

Proposal

Add a Close method to BaseApp which is called in server start cmd, and the custom app can override.

[alexanderbez]

Concept ACK 👍