GH-4446: Replace tomcat7-maven-plugin with jetty-maven-plugin

[erikgb]

GitHub issue resolved: #4446

Briefly describe the changes proposed in this PR:

This PR replaces the maven plugin that can be used to run server/workbench in a local development environment. I am now able to run server in standalone mode by executing mvn jetty:run from the server module, and workbench+server if running from the workbench module.

Also cleaning up some dependency issues detected by duplicate class detection on Jetty startup. There is still a problem with duplicate commons-logging classes, but that requires more delicate work to fix. Please let me know if you want the dep-changes in a dedicated PR. I have made an attempt in #4443, but IMO it makes more sense to do it here - when actually running the applications.

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• my pull request is self-contained
• I've added tests for the changes I made
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[barthanssens]

LGTM.
Maven Jetty plugin is also covered by Eclipse license, so that's fine in terms of IP