Failover targets should be chosen deterministically

Undertow task

We would like to have this bit of configurable logic, off by default, implemented in Undertow. The C implementation is clear enough, the algorithm is just one line. I believe it could be translated to Undertow mod_cluster Proxy logic.

See: https://github.com/modcluster/mod_proxy_cluster/pull/8/files

Description

Under the following conditions:
1. A request arrives for a session with an unknown route
2. A request arrives for a session with a known route in an error state, or if the intended worker is unresponsive and needs to failover

... the failover target must be chosen in a deterministic way. This is meant to mitigate the situation where concurrent requests failover to different nodes and cause cluster-wide contention for the session. Given a consistent ordering of workers (e.g. lexographically), the failover target should be chosen by something like: hash code of session id % active worker count.