CASC-207: Make loggers in final classes static

Loggers in org.jasig.cas.client.proxy.Cas20ProxyRetriever and org.jasig.cas.client.proxy.ProxyGrantingTicketStorageImpl are fields, but these classes are final. Therefore, the loggers should be static, too, to avoid creating new loggers for each instance. Additionally, since these two classes are serializable, having non-transient loggers is not ideal (and can cause problems when using slf4j).