Co-authored-by: Zhe Sun <[email protected]>

Currently clusterKey cookie is set only when an HTTP session exists.
In addition to the cookie, the key is also set as a session attribute
so that SessionSerializer can fetch it later.
However, if two or more concurrent requests are sent from the client,
all of them create a new cookie, but the session attribute gets the value
of the first one. This causes the distributed session to be written with
a wrong key and when there a server switch the session cannot be restored.
This change sets the clusterKey cookie even if there is not yet an HTTP
session, so the key is only defined during first request and remains stable
with subsequent calls; once the HTTP session is create, the key is stored
as an attribute.

Potentially fixes #111

See https://github.com/vaadin/kubernetes-kit/issues/111\#issuecomment-2074909790

Co-authored-by: Tamas Mak <[email protected]>

VaadinRequest thread local is not available during a VaadinSession.access execution.
VaadinRequest is used to access the HTTP session wrapper, that can be obtained through
VaadinSession. The most important usage is however getting the update version header,
but the lookup can be done outside the access block.
This change also removes ClusterSupport registration in CurrentInstance becuase it
seems not to be used anywhere and furhtermore the thread local is set only on the
thread that calls serviceInit and never cleaned up.

Fixes #148

Co-authored-by: Tamas Mak <[email protected]>