added CreationTimestamp to HttpHealthCheck and TargetPool + refactor War...

...ning

Moves common code for Warning and KeyValuePair our to their own files.
Adds creationTimestamp to HttpHealthCheck and TargetPool
Adds missing live tests for InstanceApi
Improves ParseHttpHealthCheckTest
Are these properties *package-private* for any particular reason? In that case, would it make sense to make the whole class package-private? (I'm not sure if that will prevent it from being deserialised properly):