refactor(dcdn): sync etc within content feed

[bplaster]

Fixes #34 Fixes #33

Proposed Changes

• Checks ddo proof to validate etc partition key
• Removes metadata.js and syncs inside of requester/farmer
• Next step could be to extract replication into replicate.js because of duplicate code

[vipyne]

Re: replicate.js is that intended to be a new file in this repo? How does that factor in to https://github.com/AraBlocks/ara-reward-dcdn/issues/58 ? Currently https://github.com/AraBlocks/ara-network-node-dcdn handles replication. Can drying up replication code (there is a pun to be made here 🙃) consider and include https://github.com/AraBlocks/ara-network-node-dcdn/issues/53 ?

[bplaster]

@vipyne Re: replicate.js Yes #58 should be kept in mind when doing this, though the replicate functionality in base will need to enable passing in hypercore extensions, as well as some sort of custom logic of when to replicate, given that currently it will freely replicate, and it uses different topics for the etc partition.