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.

Signed-off-by: Ben Evans <bevans@cray.com>
Change-Id: I563666a1a3e53f0ec5908de593de71ff4d925467
Reviewed-on: https://review.whamcloud.com/29742
Reviewed-by: Frank Zago <fzago@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Sergey Cheremencev <cherementsev@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>