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/timeoutconn/timeoutconn_readv_unsupported.go

@@ -4,7 +4,7 @@ package timeoutconn
 
 import "net"
 
-func (c Conn) readv(buffers net.Buffers) (n int64, err error) {
+func (c *Conn) readv(buffers net.Buffers) (n int64, err error) {
 	// Go does not expose the SYS_READV symbol for Solaris / Illumos - do they have it?
 	// Anyhow, use the fallback
 	return c.readvFallback(buffers)