Cleanup RPC status and peers endpoints

The concept of peers has changed, as there are no longer partiallly and fully connected peers, just “peers” (a single connection is sufficient now).

In the RPC and synchronizer components there is still the notion of these different kinds of peers (regular and fully connected peer), these should be unified to just a single peer type, which is essentially equal to the fully connected peer from before.

Some care must be taken to collect backwards-compatibility requirements from SRE and RPC leads, as these changes may affect their interaction with the system. If in doubt, the outside facing interface can be kept the same.

Example site: https://github.com/marc-casperlabs/casper-node/blob/66a5812653330f004ebd1b30e8852ca577a4eb9a/node/src/effect/requests.rs#L211-L222

This work should be based off of and merged into the https://github.com/marc-casperlabs/casper-node/blob/4525-connection-management branch.