Update DB schema and add support for overlapping

[krshrimali]

The new schema looks like:

file_path: {
    "<start_line_number>_<end_line_number>": [....],
}

Overlapping support:

Imagine a scenario where user makes query for 2nd line to 100th line - the server should process it and return the result. But next time, if the user queries for 3rd to 103rd line - the server ideally should just process for 101st, 102nd, and 103rd lines and everything else should be fetched from the DB for 2nd to 100th line. This PR addresses exactly that, and it's fixed now.

I'm hesitant to release this right now, as the code is not tested and this is a big change. But... still thinking :)