[pgAdmin4][Patch]: Ensure any changes to the config database arebackward compatible

We will use config variable `SCHEMA_VERSION` to keep track of SQLite database changes. Whenever there is any change is performed in the database, the count of schema version will be incremented by 1 and the same will be updated in SQLite table `version`.

So, these changes make sure the database migration is not performed on the older version of pgAdmin4 and this way the same database will work irrespective of pgAdmin4 version.

Also, this will only work for future pgAdmin4 versions.

Please find attached patch and review.

Thanks,

Surinder