Made sure that when compiling with JDK 9 we actually use a JDK 8 runtime.
Updated the Maven Compiler Plugin to latest version, with support for
JDK 9 --release command line.
Removed unnecessary Maven Compiler Plugin declarations in POMs.
* Updated Maven plugins to versions that support JDK 9.
* Added jdk9 profiles to the build files.
* Introduced modules jetty-alpn-java-client and jetty-alpn-java-server
containing a pure JDK 9 implementation of ALPN.
* Wired ALPN connection factories (client and server) to use the proper
ALPN implementation based on the JDK platform version (8 or 9).