Change impl in client header file to be reference counted
Change impl in client header file to be reference counted

This will allow Log objects to outlive Cluster objects, which could save
some memory corruption headaches for application developers.

gaoning777/logcabin,chaozh/logcabin,logcabin/logcabin,gaoning777/logcabin,tempbottle/logcabin,chaozh/logcabin,tempbottle/logcabin,nhardt/logcabin,jimpelton/mingle-logcabin,TigerZhang/logcabin,TigerZhang/logcabin,tempbottle/logcabin,nhardt/logcabin,TigerZhang/logcabin,chaozh/logcabin,nhardt/logcabin,dankenigsberg/logcabin,gaoning777/logcabin,jimpelton/mingle-logcabin,jimpelton/mingle-logcabin,TigerZhang/logcabin,gaoning777/logcabin,logcabin/logcabin,chaozh/logcabin,tempbottle/logcabin,logcabin/logcabin,dankenigsberg/logcabin,nhardt/logcabin,dankenigsberg/logcabin,dankenigsberg/logcabin,logcabin/logcabin