Deprecated libraries in favor of abstract level

[tobiasBora]

When running:

$ npm i pouchdb

I get many warnings:

npm warn deprecated level-concat-iterator@2.0.1: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated encoding-down@6.3.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated level-errors@2.0.1: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated level-js@5.0.2: Superseded by browser-level (https://github.com/Level/community#faq)
npm warn deprecated level-concat-iterator@3.1.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated deferred-leveldown@5.3.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated level-packager@5.1.1: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated level-codec@9.0.2: Superseded by level-transcoder (https://github.com/Level/community#faq)
npm warn deprecated levelup@4.4.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated abstract-leveldown@6.2.3: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated abstract-leveldown@7.2.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated leveldown@6.1.1: Superseded by classic-level (https://github.com/Level/community#faq)
npm warn deprecated leveldown@5.6.0: Superseded by classic-level (https://github.com/Level/community#faq)

Would be great to fix these dependencies.

[chrisjdev]

I'm guessing it may not be a priority if they're removing level completely, but I think it'd be an improvement to remove leveldown from PouchDB's package.json. As indicated in the warnings, there's 2 different leveldown warnings, and I think the version conflict makes it more challenging when used in Electron, to build for multiple platforms.