p2p/conn: FlushStop. Use in pex. Closes #2092 (#2802)

* p2p/conn: FlushStop. Use in pex. Closes #2092

In seed mode, we call StopPeer immediately after Send.
Since flushing msgs to the peer happens in the background,
the peer connection is often closed before the messages are
actually sent out. The new FlushStop method allows all msgs
to first be written and flushed out on the conn before it is closed.

* fix dummy peer

* typo

* fixes from review

* more comments

* ensure pex doesn't call FlushStop more than once

FlushStop is not safe to call more than once,
but we call it from Receive in a go-routine so Receive
doesn't block.

To ensure we only call it once, we use the lastReceivedRequests
map - if an entry already exists, then FlushStop should already have
been called and we can return.

blockchain/reactor_test.go

@@ -197,6 +197,7 @@ func (tp *bcrTestPeer) TrySend(chID byte, msgBytes []byte) bool {
 	return true
 }
 
+func (tp *bcrTestPeer) FlushStop()                           {}
 func (tp *bcrTestPeer) Send(chID byte, msgBytes []byte) bool { return tp.TrySend(chID, msgBytes) }
 func (tp *bcrTestPeer) NodeInfo() p2p.NodeInfo               { return p2p.DefaultNodeInfo{} }
 func (tp *bcrTestPeer) Status() p2p.ConnectionStatus         { return p2p.ConnectionStatus{} }