Add a metadata object parameter for rados bench

rados bench is useful to benchmark the cluster, however, there is one user case it does not support: read after write if there are multiple write instance, this is because the current implementation hard code the metadata object which will make the last update win, as a result, the metadata of the previous run is lost and user is unable to run a read (seq) based on that round of write.

This fix make things backward compatible, and if user has a request like us (run read after multiple write), user can specify a new meta-object to record the metadata.

Signed-off-by:  Guang Yang (yguang@yahoo-inc.com)