Git external repo fixes:
- re-enabled polling logic in case of git external repo
- unified configuration constants.
- fixes #2514
Clean use of maven
  * put aether/maven in its own bundle
  * use it for agent and proxy
  * remove old modules (insight-maven, maven-util)
Delete deprecated fab
Abstract a bit more the use of the MavenResolver
Export the MavenResolver as a service, use it for the maven proxy, trim down a bit the dependencies of fabric-maven