Offload toTabs() computation to a worker thread

[MarcLeclair]

Why/User Benefit/User Problem

The more tabs we have in our UI, the longer it takes to start our app ( as well as rendering our View Holders). For instance, running ~10 tabs and opening Fenix will take an extra 2% on a Moto G5 on a regular start up. So this means ~ 148 ms for HomeFragment.onStart instead of 40 in the current set up. There's also been random cases where I got really high number from toTab around 700 ms but I can't reproduce it, so I'm still trying to figure out what the culprit was there.

*Keep in mind, my evaluation of off loading the toTabs computation on a separate thread was a quick implementation, there might be better ways to do it.

** This also seems to help performance throughout the app on the HomeFragment, but I noticed that quickly as a by-product of my implementation, so more research should go into that

Impact

For a user that keeps a lot of tabs open (i.e, opening links from external apps will pile on your tabs -- I'm guilty of that), the load time can get considerably longer.

Acceptance Criteria (how do I know when I’m done?)

Start up time shouldn't be affected by the number of tabs open, especially the string computation that comes along with toTabs. Honnestly, the thing that takes the most time in here is the toShortURL

*** I will add my traces later, I just need to order them so they are clearly labeled

┆Issue is synchronized with this Jira Task

[mcomella]

Triage: we're concerned whether the potential improvements are worth the risks.

@ecsmyth mentioned they'll look into the telemetry on average number of tabs open.

[NotWoods]

We could also pass Sessions to our RecyclerView and compute toTab only when the tab is visible on the screen.

[mcomella]

Moving to In Progress so devs don't accidentally take this (action is on Eric).

[ecsmyth]

Given the change to the design of the home screen (6190), I think this issue can return to the backlog for now.

[mcomella]

Closing: we're addressing startup holistically and I added this to our start up spreadsheet to track it.