compaction_manager: fix crash when dropping a resharding column family
compaction_manager: fix crash when dropping a resharding column family

Problem is that column family field of task wasn't being set for resharding,
so column family wasn't being properly removed from compaction manager.
In addition to fixing this issue, we'll also interrupt ongoing compactions
when dropping a column family, exactly like we do with shutdown.

Fixes #2291.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <20170418125807.7712-1-1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

scylladb/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla