Add shutdown hook - via DataSourcePool.builder().shutdownOnJvmExit(true)

[rbygrave]

With shutdownOnJvmExit(true) then a JVM shutdown hook is registered and when the JVM exists then it will run and shutdown the DataSourcePool (unless it has already been shutdown).

 DataSourcePool pool = DataSourcePool.builder()
        .name("test")
        .url("jdbc:h2:mem:test"i)
        .username("sa")
        .password("")
        .shutdownOnJvmExit(true)
        .build();

Note that this PR leaves the default for this to be off by default. This might change to default to on by default.