feat: allow control over uploading to peers

[gmaclennan]

Partial fix to #305

Creating as a draft for initial feedback on proof-of-concept.

This is useful because you may want to download data from a peer, but not allow uploads to a peer.

@mafintosh would one of your team be able to look at this? I'm a bit out of my depth with the inner workings of the replicator code.