Reduce GC pressure of ConditionMessage creation

[wilkinsona]

The creation of condition messages performs a reasonable amount of string concatenation that's contributing to GC pressure during startup. Without Actuator and --debug, the messages will never actually be read. It might be interesting to investigate computing the messages lazily. It may not be straightforward as the message String is currently used in hashCode and equals implementations.

[wilkinsona]

In a minimal web app, the condition evaluation report adds ~140KB to the heap with no benefit. Aside from the GC pressure mentioned above, we could consider making the report a no-op if it's not going to be logged or if you haven't got Actuator on the classpath.