By default, Tomcat forces the generation of a session id during startup
to ensure that a SecureRandom instance has been initialized. When there
is a lack of entropy (as is often the case on a newly booted VPS, for
example) this can block for a long time (several minutes in some cases)
causing users to incorrectly believe that their application has hung
during startup. This is particularly problematic for applications that
don't use HTTP sessions as they are paying the startup cost for no
benefit.

This commit address the problem by configuring a custom
SessionIdGenerator that does not initialize itself during startup.
Instead, the initialization is now deferred until a request for a
session id is made.

Closes gh-6174