Mesh/peer abstractions don't have a way to express only-send or only-receive relationships for sync

[micahscopes]

[sbazerque]

At the moment, the Peer & PeerGroup abstractions are unaware of the gossip / sync layers that are built on top of them.

A member of a peer group can either send messages to another peer or to all peers it is currently connected to within the group. Gossip and sync are built on that abstraction. The "sharing" and "fetching" parts are two different modules within the sync agent, so it would be possible to configure the agent to do only one of the two functions. The part that seems more cumbersome is indicating which peer is in which mode with respect to each sync target.