Implement mod_cluster worker for Undertow embedded and standalone use case

This is a placeholder JIRA for a possible implementation/artifact. It has neither any formal planning nor priority set and it might not happen at all.

Workers and balancers

Mod_cluster project comprises two kinds of actors: balancers and workers.

Balancers

Currently there are three balancer implementations:

1. mod_proxy_cluster module for Apache HTTP Server
2. Undertow mod_cluster filter that could be used both in Undertow embedded and in Wildfly application server
3. mod_proxy_cluster Nginx implementation

Workers

There are these worker implementations:

1. JBoss AS 5 / mod_cluster.sar worker implementation, Abandoned/Legacy
2. JBoss AS7 mod_cluster subsystem, Legacy
3. Wildfly application server, Current
4. Tomcat 6.0/7.0/8.0, Legacy
5. Tomcat 8.5/9.0, Current
   In addition to those, we would like to see mod_cluster worker login implemented for additional web servers, such as:
6. Jetty
7. Go net/http, i.e. Go web servers (mostly for REST APIs)
8. Undertow embedded
9. Undertow standalone (UNDERTOW-1356)

Undertow worker implementation

There should be a mod_cluster worker library that would enable Undertow, both in its embedded and standalone (UNDERTOW-1356) flavour to act as a worker, i.e. to report to another mod_cluster balancer (either httpd, Nginx or another Undertow instance).