Apply server.jetty.* config to reactive servers

This commit applies `server.jetty.*` configuration properties
to Jetty when configured as a reactive web server.

It also removes some infrastructure support for Jetty 8, which
is not supported anymore in Spring Boot 2.0 (partial fix for
gh-11504).

See gh-11500

spring-boot-project/spring-boot-parent/src/checkstyle/import-control.xml

@@ -14,6 +14,9 @@
 			<subpackage name="client">
 				<allow pkg="org.springframework.boot.web.client" />
 			</subpackage>
+			<subpackage name="embedded.jetty">
+				<allow pkg="org.springframework.boot.web.embedded.jetty" />
+			</subpackage>
 			<subpackage name="embedded.tomcat">
 				<allow pkg="org.springframework.boot.web.embedded.tomcat" />
 			</subpackage>