Moving to non-persistent background script

Currently, the extension is persistent so it always runs even if nothing is happening. As a result, everything is a global variable.

Move it to non-persistent. Tried doing it but turned out to be trickier than I thought. Here is the migration guide -
https://developer.chrome.com/extensions/background_migration

@chetdeva this needs a serious architecture thinking (currently there is none - thanks to global variables) so let us jam and feel free to dump your thought.

Start with Chrome's background migration guide and inspect the extension's background script and its global variables. Map which state currently depends on the persistent process before proposing the architecture. Done means the extension runs as a non-persistent background script without relying on process-lifetime globals.

Written by the indexing model from the issue text.