server: add new peers within OnVerAck instead of within OnVersion
server: add new peers within OnVerAck instead of within OnVersion

This change is needed as part of requiring peers to also send a verack
message following their version message during protocol negotiation.
Peers were previously added to the SyncManager before their message
queues were started, causing the server to stall if a peer didn't
provide a timely verack response following their version. We now do this
within OnVerAck, which happens shortly before peer message queues are
started.

dajohi/btcd,btcsuite/btcd,ltcsuite/ltcd,btcsuite/btcd,dajohi/btcd,davecgh/btcd,tb00/btcd,tb00/btcd,davecgh/btcd,ltcsuite/ltcd,dajohi/btcd