Add Support for Cancelling Retries

[rgmz]

Motivation:

This fixes #46.

Conformance:

Your commits should be signed and you should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines

[rgmz]

@vietj Here's my first pass at #46, let me know your thoughts.

There is an issue resolving the dependencies for the latest snapshot (hence Travis CI failure), but it works if you switch back to an older version:

diff --git a/pom.xml b/pom.xml
index 8b9ffae..b7afb9f 100644
--- a/pom.xml
+++ b/pom.xml
@@ -23,7 +23,7 @@
   </parent>
 
   <artifactId>vertx-circuit-breaker</artifactId>
-  <version>3.9.2-SNAPSHOT</version>
+  <version>3.9.0</version>

Also, I wasn't sure if I should use master or 3.9 as a base. I went with the latter, but I can switch it to master.

[rgmz]

The meat of the changes are in the first commit (https://github.com/vert-x3/vertx-circuit-breaker/pull/47/commits/8e3d35e7dfb81959cb13507286ce38bbbbff81e5), https://github.com/vert-x3/vertx-circuit-breaker/pull/47/commits/b8efb6835c4f4c08c626bfdd529fe0449c556cfc just provides some enhancements.