[BackEnd] last_active_timestamp is null for tab not opened yet, it interfers with behaviour

[MerlinLaffitte]

What

• Before setting the invulnerability feature we add a not orthodox behaviour : newly created tab were the first to be shutdown (same for restored tab)

How

• Set a new created_at attribute for tabs in order to handle deletion on max(last_active_timestamp ,created_at)

Why

• We still can have a strange behaviour when the protection time expires

[faameunier]

There is already a basic protection here: https://github.com/faameunier/tk-chrome/blob/93a60e6f5ad3f92bab761f487e8ddce3a6365f84/src/services/scorer.js#L45-L50

It definitely can be improved (maybe add a exp decay in the score from max score to 1 ?) Same applies for protection after restore or next ?

We could also move these logic to the abstract class as I don't think it will change depending on the scorer, but that's for a V10

[faameunier]

closing this as last_active_timestamp is now initiated at creation time and when restored (PR #63)