rpc: fix data race in timeoutconn

- `timeoutconn` handles state, yet calls to Read/Write make a copy of
  that state (non-pointer receiver) so any outbound calls will not have
  the state updated

- Even without the copy issue, the renew methods can in edge cases set a
  new deadline _after_ DisableTimeouts have been called, consider the
  following racy behavior:
    1. `renewReadDeadline` is called, checks `renewDeadlinesDisabled`
       (not disabled)
    2. `DisableTimeouts` is called, sets `renewDeadlinesDisabled`
    3. `DisableTimeouts` invokes `c.SetDeadline`
    4. `renewReadDeadline` invokes `c.SetReadDeadline`

To fix the above, the `Conn` receiver was made to be a pointer
everywhere and access to renewDeadlinesDisabled is now guarded
by an RWMutex instead of using atomics.

closes #415

rpc/dataconn/frameconn/frameconn.go

@@ -47,7 +47,7 @@ func (f *FrameHeader) Unmarshal(buf []byte) {
 
 type Conn struct {
 	readMtx, writeMtx sync.Mutex
-	nc                timeoutconn.Conn
+	nc                *timeoutconn.Conn
 	readNextValid     bool
 	readNext          FrameHeader
 	nextReadErr       error
@@ -55,7 +55,7 @@ type Conn struct {
 	shutdown          shutdownFSM
 }
 
-func Wrap(nc timeoutconn.Conn) *Conn {
+func Wrap(nc *timeoutconn.Conn) *Conn {
 	return &Conn{
 		nc: nc,
 		//		ncBuf: bufio.NewReadWriter(bufio.NewReaderSize(nc, 1<<23), bufio.NewWriterSize(nc, 1<<23)),