Eliminate shared ClientExecutor from ClientRequest

If an org.jboss.resteasy.client.ClientRequest is created without explicitly being passed a ClientExecutor, it will create one, based on the value of ClientRequest.defaultExecutorClasss. Currently, there is a non-default option to create a single shared instance of defaultExecutorClasss, which will be returned by ClientRequest.getDefaultExecutor(). The option can be turned on by calling the static method

public static void setDefaultExecutorClass(String classname, boolean createPerRequestInstance)

with createPerRequestInstance set to false.

This option will be removed.