Fix Checkstyle violations

* Add `validateOnlyOverlapping = false` for `RequireThis` Checkstyle rule
* Add `this.` to caught places
* Use `CustomizableThreadFactory` instead of raw interface in the
`UnicastSendingMessageHandler`

src/checkstyle/checkstyle.xml

@@ -68,6 +68,7 @@
 		</module>
 		<module name="MultipleVariableDeclarations" />
 		<module name="RequireThis">
+			<property name="validateOnlyOverlapping" value="false" />
 			<property name="checkMethods" value="false" />
 		</module>
 		<module name="OneStatementPerLine" />