LU-8030 hsm: prevent duplicated HSM requests

LU-8030 hsm: prevent duplicated HSM requests

It is possible to insert identical HSM requests in the HSM action
list. The following command

  lfs hsm-archive foo foo foo foo

results in 4 new archive requests in the "actions" list for the same
file.

Identify such requests and suppress the duplicates.

Signed-off-by: frank zago <fzago@cray.com>
Change-Id: If83d2855948578b94babca96dc72c33f96ae4f71
Reviewed-on: http://review.whamcloud.com/19635
Tested-by: Jenkins
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>