Require default event ExecutorService to be managed on Java EE

When running on a Java EE environment, CDI should use a managed executor service by default for asynchronous operations.

This is already required by the JAX-RS 2.1 spec (http://download.oracle.com/otndocs/jcp/jaxrs-2_1-pfd-spec/index.html), section 5.8:

In an environment that supports the Concurrency Utilities for Java EE [13], such as the Java EE Full Profile, implementations MUST use ManagedExecutorService and ManagedScheduledExecutorService, respectively. The reader is referred to the Javadoc of ClientBuilder for more information about executor services.

Containers will presumably offer monitoring features and thread pool configuration options for managed executor services.