p2p: use fake net.Pipe since only >=Go1.10 implements SetDeadline

Fixes https://github.com/tendermint/tendermint/issues/851

Go1.9 and below's net.Pipe did not implement the SetDeadline
method so after commit
e2dd8ca946
this problem was exposed since now we check for errors.

To counter this problem, implement a simple composition for
net.Conn that always returns nil on SetDeadline instead of
tripping out.

Added build tags so that anyone using go1.10 when it is released
will be able to automatically use net.Pipe's net.Conns

p2p/switch.go

@@ -515,7 +515,7 @@ var PanicOnAddPeerErr = false
 func Connect2Switches(switches []*Switch, i, j int) {
 	switchI := switches[i]
 	switchJ := switches[j]
-	c1, c2 := net.Pipe()
+	c1, c2 := netPipe()
 	doneCh := make(chan struct{})
 	go func() {
 		err := switchI.addPeerWithConnection(c1)
@@ -593,3 +593,27 @@ func (sw *Switch) addPeerWithConnectionAndConfig(conn net.Conn, config *PeerConf
 
 	return nil
 }
+
+// Only Go1.10 has a proper net.Conn implementation that
+// has the SetDeadline method implemented as per
+//  https://github.com/golang/go/commit/e2dd8ca946be884bb877e074a21727f1a685a706
+// lest we run into problems like
+//  https://github.com/tendermint/tendermint/issues/851
+// so for go versions < Go1.10 use our custom net.Conn creator
+// that doesn't return an `Unimplemented error` for net.Conn.
+// Before https://github.com/tendermint/tendermint/commit/49faa79bdce5663894b3febbf4955fb1d172df04
+// we hadn't cared about errors from SetDeadline so swallow them up anyways.
+type pipe struct {
+	net.Conn
+}
+
+func (p *pipe) SetDeadline(t time.Time) error {
+	return nil
+}
+
+func netPipe() (net.Conn, net.Conn) {
+	p1, p2 := net.Pipe()
+	return &pipe{p1}, &pipe{p2}
+}
+
+var _ net.Conn = (*pipe)(nil)