Add support for keepAlive

[weiwei-lin]

Feature request
Support equivalent of @computed({keepAlive: true}).
Note that keepAlive from mobx-utils isn't the same. Since it causes the computed value to always be re-evaluated even there's no other observers.

Is your feature request related to a problem? Please describe.

The lack of computed support make the friction of migrating from MobX to MST a lot higher.
And given that MST use a tree-like structure for managing states, arguably the chance of memory leak is even lower.

Are you willing to (attempt) a PR?

• Yes
• No

[coolsoftwaretyler]

@weiwei-lin - kind of like I mentioned in #1950 - are you still having trouble with this/interested in opening a PR? I don't know if I understand all the implications of this feature request, but I'd be happy to talk with you about it if it's still on your mind all this time later.