LU-11270 mdt: better DOM data discard

LU-11270 mdt: better DOM data discard

Discard DOM data when all object are put and unlocked.
So current thread will wait for discard but resources are
not blocked by it. Also cancel discard lock immediately.

Patch also add missing part for lock prolongation for DoM
locks and fix for tgt_blocking_ast() to don't miss a call to
ldlm_server_blocking_ast() in case of lu_env_init() failure.

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I09aaa08e28d2a280ca1f9939315311c45d0334b5
Reviewed-on: https://review.whamcloud.com/33026
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>