maintenance_manager: schedule work immediately when threads are free
maintenance_manager: schedule work immediately when threads are free

This changes the MM so that, when a worker thread becomes available, it
immediately wakes up the scheduler to schedule the next available work item.
Additionally, the scheduler will loop scheduling new items as long as there
are free worker threads, instead of only scheduling once per polling
interval.

Change-Id: I63c4b48f5f02f3a1d3a8964993e78037ce72b1da
Reviewed-on: http://gerrit.cloudera.org:8080/6815
Reviewed-by: David Ribeiro Alves <dbbafdb4f25eb0c1ff3facf0e5f2f27705055af1@gmail.com>
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

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