etcd: add a read/write timeout to server

The default is for connections to last forever[1]. This leads to fds
leaking. I set the timeout so high by default so that watches don't have
to keep retrying but perhaps we should set it slower.

Tested on a cluster with lots of clients and it seems to have relieved
the problem.

[1] https://groups.google.com/forum/#!msg/golang-nuts/JFhGwh1q9xU/heh4J8pul3QJ

server/listener.go

@@ -3,10 +3,16 @@ package server
 import (
 	"crypto/tls"
 	"net"
+	"time"
 
 	"github.com/coreos/etcd/log"
 )
 
+const (
+	DefaultReadTimeout  = float64((5 * time.Minute) / time.Second)
+	DefaultWriteTimeout = float64((5 * time.Minute) / time.Second)
+)
+
 // TLSServerConfig generates tls configuration based on TLSInfo
 // If any error happens, this function will call log.Fatal
 func TLSServerConfig(info *TLSInfo) *tls.Config {