Reconsider necessity of putting tab activity state in sessionstore.

[sfleiter]

I asked myself why the tab activity is persisted in sessionstore.
The only reason that came to my mind is extension updates.
If there is no other reason the size of the sessionstore and generated write load be reduced by collecting the data in memory.

For extension updates the data could be persisted in runtime.onUpdateAvailable and loaded in init.

I have some code for that already if desired.
Relates to issue #35.