Add @UsesUnsafeJava annotation

Add a new annotation to indicate to Animal Sniffer that a method uses
a call to `Unsafe`.

Closes gh-3080

spring-boot-parent/pom.xml

@@ -513,6 +513,7 @@
 					<annotations>
 						<annotation>org.springframework.lang.UsesJava8</annotation>
 						<annotation>org.springframework.lang.UsesJava7</annotation>
+						<annotation>org.springframework.boot.lang.UsesUnsafeJava</annotation>
 					</annotations>
 				</configuration>
 				<executions>