Fix missing argument on next tick of dbCleanup

[jkiyo]

This PR fixes a problem I encountered when the next tick of dbCleanup runs.

(node:4432) UnhandledPromiseRejectionWarning: TypeError: Cannot set property 'nextDbCleanup' of undefined
    at C:\Dev\sample_app\node_modules\connect-session-knex\index.js:242:31
    at <anonymous>
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:4432) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(
). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:4432) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

--- Edit ---

More debug details:

The KnexStore is defined on the first run

But when the next tick runs from the scheduled setTimeout the KnexStore is undefined:

[omarryhan]

Whoops, must've missed that in my last PR. Thank you for contributing :)