compaction_manager: dont go on with major compaction if task was stopped
compaction_manager: dont go on with major compaction if task was stopped

A column family which was truncated will remove itself from compaction
manager. Any task running a compaction should be interrupted and a
task waiting to run should bail out when it wakes up.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <20170425224350.15965-3-1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

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