Correct the "is peer live" checks in `PeerManager`

Correct the "is peer live" checks in `PeerManager`

In general, we should be checking if a `Peer` has `their_features`
set as the "is this peer connected and have they finished the
handshake" flag as it indicates an `Init` message was received.

While none of these appear to be reachable bugs, there were a
number of places where we checked other flags for this purpose,
which may lead to sending messages before `Init` in the future.

Here we clean these cases up to always use the correct check (via
the new util method).