Set a user defined hash method; if no hash method is specified,
a default hash method is used.
Since no hash method performs equally well on all possible data,
the user may find that the built-in hash method performs poorly with
a particular data set.
User specified hash functions must take a pointer to a byte string and
a length as arguments and return a value of type u_int32_t.
If a hash method is specified, hash_open will attempt to determine
if the hash method specified is the same as the one with which the database
was created, and will fail if it detects that it is not.