Add SslHandler.renegotiate()

Related: #3125

Motivation:

We did not expose a way to initiate TLS renegotiation and to get
notified when the renegotiation is done.

Modifications:

- Add SslHandler.renegotiate() so that a user can initiate TLS
  renegotiation and get the future that's notified on completion
- Make SslHandler.handshakeFuture() return the future for the most
  recent handshake so that a user can get the future of the last
  renegotiation
- Add the test for renegotiation to SocketSslEchoTest

Result:

Both client-initiated and server-initiated renegotiations are now
supported properly.