Currently, the logic around whether we called CAddrMan::Connected() for
a peer is spread between verack processing (where we discard inbound
peers) and FinalizeNode (where we discard misbehaving and
block-relay-only peers). Consolidate that logic to a single place.
Also remove the CNode.fCurrentlyConnected bool, which is now
redundant. We can rely on CNode.fSuccessfullyConnected, since the two
bools were only ever flipped to true in the same place.