Improve walking algorithm to be more efficient

[TanaTanoi]

It probably doesn't need to walk the entire document, investigate and find out what specific classes we can search for to just walk across messages? jquery is probably faster

[pdonias]

This seems pretty consistent on my side:

• main panel:
  • sent and received: span._3oh._58nk
• left side panel:
  • sent: em._4qba > span
  • received: span._1htf > span
• messages notifications dropdown:
  • sent and received: div.snippet.preview > span > span:not(._j0r)
• small chat box:
  • sent and received: span._5yl5 > span

Note that the inner span can be wrapped in an extra span if there is a link, an emoji, etc... Let's wait and see if it is actually consistent and if it doesn't change over time.