hash-func: change value type of string_hash_ops_free_free to void*

hash-func: change value type of string_hash_ops_free_free to void*

The generic string_hash_ops_free_free hash operations vtable currently
assumes the data pointer is of type char*. There's really no reason to
assume that though, we regularly store non-string data as value in a
hashmap. Hence, to accomodate for that, use void* as pointer for the
value (and keep char* for the key, as that's what
string_hash_ops_free_free is for, after all).