LU-7988 hsm: wake up cdt when requests are empty

LU-7988 hsm: wake up cdt when requests are empty

The coordinator only runs once per second, we need a mechanism
to send more work when everything is done (cdt_request_count
goes to zero)

Without this, there is a hard limit of max_requests per sec
requests that can be processed, causing performance issues
with small files.

Lustre-change: https://review.whamcloud.com/29742
Lustre-commit: 7251fea8dc3c4d29e30c5a3f763c4c33d35f90a7

Signed-off-by: Ben Evans <bevans@cray.com>
Change-Id: I563666a1a3e53f0ec5908de593de71ff4d925467
Reviewed-by: Frank Zago <fzago@cray.com>
Reviewed-by: Sergey Cheremencev <cherementsev@gmail.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/30538
Tested-by: Jenkins
Reviewed-by: Sergey Cheremencev <c17829@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>