client: Fix timeout reset during TLS handshake (go-mysql-org#925)

This commit fixes `(*client.Conn).writeAuthHandshake()` to use
`packet.NewConnWithTimeout` instead of `packet.NewBufferedConn`
when recreating the packet connection after switching TLS on.
This preserves the connection read/write timeout settings which
would otherwise be reset to zero.

Since this code executes after some reads and writes have already
taken place, and the packet connection code only sets a deadline
when the timeout values are nonzero, the result was that previously
when connecting using TLS and with a read and/or write timeout set,
the connection would inevitably fail just one timeout-duration after
being opened.

This use of `packet.NewBufferedConn` appears to be the only place
in the `client` package where a packet connection was recreated
without the timeout configuration being plumbed through.

client/auth.go

@@ -304,7 +304,7 @@ func (c *Conn) writeAuthHandshake() error {
 		}
 
 		currentSequence := c.Sequence
-		c.Conn = packet.NewBufferedConn(tlsConn, c.BufferSize)
+		c.Conn = packet.NewConnWithTimeout(tlsConn, c.ReadTimeout, c.WriteTimeout, c.BufferSize)
 		c.Sequence = currentSequence
 	}