Refactor to lazy Environment creation where possible

This commit avoids eager creation of Environment instances, favoring
delegation of already existing Environment objects where possible. For
example, FrameworkServlet creates an ApplicationContext; both require
a StandardServletEnvironment instance, and prior to this change, two
instances were created where one would suffice - indeed these two
instances may reasonably be expected to be the same. Now, the
FrameworkServlet defers creation of its Environment, allowing users to
supply a custom instance via its #setEnvironment method (e.g. within a
WebApplicationInitializer); the FrameworkServlet then takes care to
delegate that instance to the ApplicationContext created
in #createWebApplicationContext.

This behavior produces more consistent behavior with regard to
delegation of the environment, saves unnecessary cycles by avoiding
needless instantiation and calls to methods like
StandardServletEnvironment#initPropertySources and leads to better
logging output, as the user sees only one Environment created and
initialized when working with the FrameworkServlet/DispatcherServlet.

This commit also mirrors these changes across the corresponding
Portlet* classes.

Issue: SPR-9763

spring-web/src/main/java/org/springframework/web/context/support/AbstractRefreshableWebApplicationContext.java

@@ -134,7 +134,8 @@ public abstract class AbstractRefreshableWebApplicationContext extends AbstractR
 	}
 
 	/**
-	 * Create and return a new {@link StandardServletEnvironment}.
+	 * Create and return a new {@link StandardServletEnvironment}. Subclasses may override
+	 * in order to configure the environment or specialize the environment type returned.
 	 */
 	@Override
 	protected ConfigurableEnvironment createEnvironment() {