lf_hash get performance regression since the bucket size won't decrease

The rw_trx_hash is implement by lf_hash, After running oltp_write_only with 1024 threads sysbench, there is lots of trx_id insert into lf_hash, lf_hash will increase it's bucket size. However, after finish the sysbench, and running cleanup operation, the item will been deleted in lf_hash, however the bucket size won't decrease, there is still lots of dummy nodes in it.

After that running oltp_read_write sysbench with 256 thread, the performance will get regression compare to running oltp_read_write sysbench directly without runing oltp_write_only with 1024 threads, even we have clean the data.

in the test case, we can find that the lf_hash size is 512, however, there is only 2 item in it, so in the iterator operation, we need to iterator 512 dummy node to get the 2 item, that cause the performance regression.

so can we add a new operation to reduce the bucket size and delete dummy node.