[backport 7.9] Improve and fix edge cases in ChannelManager - HST handshake

The implementation of the handshake between the ChannelManager and HST in the Main class has a few wrinkels and bugs (blank page or even NPE in case of invalid/missing parameters).
Furthermore the code for determining the farthest request host (looking at X-Forwarded-Host header) can be more centralized from the RestProxyServicePlugin and then also reused in the Main class as well.