Add unified NextProtoNego extension support to SslContext

Motivation:

- OpenSslEngine and JDK SSLEngine (+ Jetty NPN) have different APIs to
  support NextProtoNego extension.
  - It is impossible to configure NPN with SslContext when the provider
    type is JDK.

Modification:

- Implement NextProtoNego extension by overriding the behavior of
  SSLSession.getProtocol() for both OpenSSLEngine and JDK SSLEngine.
  - SSLEngine.getProtocol() returns a string delimited by a colon (':')
    where the first component is the transport protosol (e.g. TLSv1.2)
    and the second component is the name of the application protocol
- Remove the direct reference of Jetty NPN classes from the examples
- Add SslContext.newApplicationProtocolSelector

Result:

- A user can now use both JDK SSLEngine and OpenSslEngine for NPN-based
  protocols such as HTTP2 and SPDY