[10.1.x] PaxWebSessionIdManager.getSessionIdSuffix misbehavior

[grgrzybek]

Backport of #2110

[grgrzybek]

There's something too different between ee10 and ee8...

[grgrzybek]

Jetty 12.1.3 upgrade in Pax Web 10 (ee8) failed with strange errors (in default servlet, imagine...), so I'm postponing this to 10.1.0.