kvs: internal KVS cache code cleanup

[chu11]

in the kvs cache we have

struct cache_entry {
<snip>
    bool data_valid;          /* flag indicating if data set, don't use
                             * data == NULL as test */
<snip>    
    uint8_t dirty:1;
};

We should use bool or uint8_t consistently for these two values. In addition, functions should return bool or int consistently for these values.