Switch use of IndexedDB openCursor to openKeyCursor for fetching keys

[MeMark2]

First off, first time contributing to this project. Let me know if I'm doing anything wrong.

A little bit of background:
I'm running into a problem with some of my production IndexedDB databases which seems to be related to accessing the whole db with openCursor. I haven't been able to figure out why I'm having this problem or how to reproduce it with a db that doesn't contain sensitive information, but I did notice that using openKeyCursor for the key and keys calls (in the IndexedDB driver) lets me work around the problem for now.

Is there a particular reason we're using openCursor instead of openKeyCursor for the key and keys call in the IndexedDB driver? Is there compatibility issues I'm not aware of? As far as I know, using openKeyCursor in this case would be much faster and is just more appropriate since we don't need any actual values for these calls.

[lincolnthree]

@MeMark2 @tofumatt I've been testing this PR both locally, and now deployed to test, and yesterday to production. It seems this has settled some of the IDBExceptions that we've been seeing with Safari on iOS 13.

[f]

Confirmed. This seems to be working.

[tofumatt]

I figured it could have been a compatibility issue, but the two APIs don't seem to differ in their platform support. So honestly: I'm not quite sure why that API was used to get keys. Might've just been that we originally (years ago) used it to get keys and never improved on it. A lot of localForage was written over five years ago, and I barely remember what I ate for lunch yesterday! 😅

I'm reviewing this PR now (I was away last week), thanks for your patience! 🙂

[tofumatt]

This seems straightforward, passes tests, should improve performance, and fixes bugs. Yay! 🎉

I'm going to rebuild the dist files and push to this branch, get this merged into master, and then push a release. Thank you very much for the PR! 🙏

[tofumatt]

Just an update that this has been published on npm as is available in 1.7.4. Thanks again for the PR 😄

[zhouxiaoping]

Which versions of Edge does this happen with? Could you file an issue for this so we can address it specifically?

userAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36 Edge/18.18363

merge request for Edge18 compatibility：#971