Close sequalize connection in stopWebServer

[mikicho]

We get:

Force exiting Jest: Have you considered using `--detectOpenHandles` to detect async operations that kept running after all tests finished?

Probably because we don't close the connection to the DB.
It may require using only one instance of sequalize.

[rubengmurray]

Seeing something similar when I run locally

A worker process has failed to exit gracefully and has been force exited. This is likely caused by tests leaking due to improper teardown. Try running with --detectOpenHandles to find leaks.