Avoid triggering type cache pollution on VertxContext.isDuplicatedContext

[Sanne]

This is a strong performance enhancement.
Using @franz1981's new agent we can now identify suboptimal code in our libraries when doing instanceof (or comparable, such as casts) operations which trigger JDK-8180450.

It would seem we have one of such cases in method VertxContext.isDuplicatedContext , the related portion of the report being:

--------------------------
2:	io.vertx.core.impl.DuplicatedContext
Count:	24323910
Types:
	io.vertx.core.impl.ContextInternal
	io.vertx.core.Context
Traces:
	io.smallrye.common.vertx.VertxContext.isDuplicatedContext(VertxContext.java:114)
		class: io.vertx.core.Context
		count: 11809285
	io.smallrye.common.vertx.VertxContext.isDuplicatedContext(VertxContext.java:115)
		class: io.vertx.core.impl.ContextInternal
		count: 10516203
	io.vertx.core.http.impl.Http1xServerConnection.handleMessage(Http1xServerConnection.java:135)
		class: io.vertx.core.impl.ContextInternal
		count: 509081
	io.quarkus.vertx.core.runtime.VertxCoreRecorder$14.runWith(VertxCoreRecorder.java:561)
		class: io.vertx.core.impl.ContextInternal
		count: 505264
	io.vertx.ext.web.impl.RoutingContextImpl.getEndHandlers(RoutingContextImpl.java:508)
		class: io.vertx.core.impl.ContextInternal
		count: 494211
	io.vertx.core.streams.impl.InboundBuffer.<init>(InboundBuffer.java:95)
		class: io.vertx.core.impl.ContextInternal
		count: 489866
--------------------------

As one can see, the common pattern is to cast an instance of io.vertx.core.impl.ContextInternal to io.vertx.core.impl.ContextInternal; there's only one exception here perturbing the cache: this method is implicitly casting to its obvious interface io.vertx.core.Context.

The fix is simple: consistently cast to io.vertx.core.impl.ContextInternal exclusively.

[franz1981]

Well done @Sanne ! LGTM

[radcortez]

@Sanne, you should be signing your commits :)