LU-10175 ldlm: handle lock converts in cancel handler

LU-10175 ldlm: handle lock converts in cancel handler

- Use cancel portals and high-priority handling for lock
  converts. Update ldlm_cancel_handler to understand
  LDLM_CONVERT RPC for that.
- Use ns_dirty_age_limit for lock convert - don't convert too old
  locks.
- Check for empty converts and skip such

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I767626acd974ad88bbbf0bb3b0a46744c45b7897
Reviewed-on: https://review.whamcloud.com/32314
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>