Set an estimate of the final size of the hash table.
If not set or set too low, hash tables will still expand gracefully
as keys are entered, although a slight performance degradation may be
noticed.
The Db::set_h_nelem interface may only be used to configure Berkeley DB before
the Db::open interface is called.
The Db::set_h_nelem method either returns a non-zero error value or throws an exception that
encapsulates a non-zero error value on failure, and returns 0 on success.