Make `vss_url` and other configs more forgiving

[npslaney]

We spent a good time chasing what we thought were issues in vss when we found out that ldk-node won't like it if you leave a trailing slash on the url :)

Maybe make this more forgiving? Or document where / how you need to be exact

[martinsaposnic]

the error was "can't read from store", and it was confusing because this was the exact same error when vss was not configured correctly (IIRC the database table was not created yet)

[TheBlueMatt]

Probably something to do on vss-server itself as well (cc @tankyleo )