legacy_schema_migrator: Use separate joinpoint instance for each table
legacy_schema_migrator: Use separate joinpoint instance for each table

Otherwise we may deadlock, as explained in commit 5e8f0efc8:

Table drop starts with creating a snapshot on all shards. All shards
must use the same snapshot timestamp which, among other things, is
part of the snapshot name. The timestamp is generated using supplied
timestamp generating function (joinpoint object). The joinpoint object
will wait for all shards to arrive and then generate and return the
timestamp.

However, we drop tables in parallel, using the same joinpoint
instance. So joinpoint may be contacted by snapshotting shards of
tables A and B concurrently, generating timestamp t1 for some shards
of table A and some shards of table B. Later the remaining shards of
table A will get a different timestamp. As a result, different shards
may use different snapshot names for the same table. The snapshot
creation will never complete because the sealing fiber waits for all
shards to signal it, on the same name.
Message-Id: <52fba011bc43edbdb0e69ea165a06819081e25a8@scylladb.com>

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