Fix IDB interface

[chuanqisun]

Fixing #126

1. Removed the constructor(...) method from the interface. When user provides a custom db instance, the constructor is never invoked by the lightning-fs so constraining it in the interface is unnecessary. Please confirm this though. In addition, if we want to constrain the constructor interface, (for whatever reason), a different setup is required. See: https://stackoverflow.com/questions/13407036/how-does-interfaces-with-construct-signatures-work
2. Renamed loadFile to readFile, matching implemention.

[jcubic]

Thanks for the PR, unfortunately I'm not able to merge because test are failing, and I don't know how to fix them.

[chuanqisun]

I just saw your bug report for karma-sauce-launcher. It should be the same issue. Just added additional info others to help debug:

1. The failing test was a headless browser test that is supposed to run on Edge v79 on a remote testing service provider (SauceLab).
2. The failure appeared to be a network issue. The remote testing service (SauceLab) refused our network connection, we have retried twice and still no connection. The actual test was never run.
3. Tests against other browsers all passed.
4. Because the issue is not related to my change, any future PRs might be blocked by the same issue.