Deprecated libraries in favor of abstract level

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npm warn deprecated level-concat-iterator@2.0.1: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated encoding-down@6.3.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated level-errors@2.0.1: Superseded by abstract-level (https://github.com/Level/community#faq)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npm warn deprecated level-concat-iterator@3.1.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated deferred-leveldown@5.3.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated level-packager@5.1.1: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated level-codec@9.0.2: Superseded by level-transcoder (https://github.com/Level/community#faq)
npm warn deprecated levelup@4.4.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated abstract-leveldown@6.2.3: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated abstract-leveldown@7.2.0: Superseded by abstract-level (https://github.com/Level/community#faq)
npm warn deprecated leveldown@6.1.1: Superseded by classic-level (https://github.com/Level/community#faq)
npm warn deprecated leveldown@5.6.0: Superseded by classic-level (https://github.com/Level/community#faq)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