BREAKING, merge after 4.0.1 - (#4222) - remove idb-alt plugin

[nolanlawson]

I am fairly certain nobody is using this. I think it was
a nice test case for a long time, and especially served
to convince us we weren't crazy when MemDOWN and LocalStorageDOWN
kept failing the tests, but level.js passed them just fine.

However, I think it's pretty clear by now that we're not going
to replace the idb adapter with this; it browserifies too large,
there are too many PouchDB-specific optimizations in the current
idb adapter, and we don't have a reasonable migration plan. So
I think we should just drop support.

This will require a major version release, so we can wait until after 4.0.1
to merge.

[nolanlawson]

Oh another reason to get rid of it: it increases the build time both in Travis and when you run npm run build.

[daleharvey]

So tentative + .5 I have always seen the levelalt adapters as experimental, I dont think they are useful alternatives at the moment but its nice to see that they work. I would be happier maybe emphasizing them in the docs and keeping them in travis. But not hugely fussed either way

[marten-de-vries]

If a moment comes where the idb-alt adapter is considered actually useful, re-enabling it shouldn't be that hard. Worst case scenario, a few bug fixes in either of the projects. The interface isn't going away. But spending time on it in the meantime? Naah. I think removing it is the right decision at the moment.

[nolanlawson]

I dont think they are useful alternatives at the moment but its nice to see that they work

Surprisingly, I'm seeing the Hoodie folks using the memory adapter everywhere; in fact they are finding bugs in IE10 that I didn't even know existed.

As for localstorage, I haven't heard much fuss, except from people who are trying to get PouchDB to run in IE9, which doesn't work because of ES3 issues. So its value is limited apparently.

For idb-alt, I have never heard a single question from a single user about it, and also it has never broken on us, most likely because it hasn't changed much over the past year.

[nolanlawson]

forgot to remove level-js from package.json

[daleharvey]

Ok lets do this - 427efbc