compaction_manager: update stat pending_tasks properly
compaction_manager: update stat pending_tasks properly

Size of both _cfs_to_cleanup and _cfs_to_compact must be added when
calculating a new value to _stats.pending_tasks.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <b601e24d0631922798575f39d00fb54fe00d4971.1457093016.git.1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

raphaelsc/scylla,kjniemi/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla