Extract schema migration into a separate class.

[rtar]

The idea is to allow several instances to exist, i.e. SetupJournalSchema and SetupSnapshotSchema.

[coveralls]

Pull Request Test Coverage Report for Build 7181790604

• 33 of 34 (97.06%) changed or added relevant lines in 2 files are covered.
• 9 unchanged lines in 8 files lost coverage.
• Overall coverage increased (+0.03%) to 81.791%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
eventual-cassandra/src/main/scala/com/evolutiongaming/kafka/journal/eventual/cassandra/MigrateSchema.scala	22	23	95.65%

Files with Coverage Reduction	New Missed Lines	%
eventual-cassandra/src/main/scala/com/evolutiongaming/kafka/journal/eventual/cassandra/CassandraSync.scala	1	86.67%
eventual-cassandra/src/main/scala/com/evolutiongaming/kafka/journal/eventual/cassandra/ReplicatedCassandra.scala	1	89.91%
eventual-cassandra/src/main/scala/com/evolutiongaming/kafka/journal/eventual/cassandra/SegmentNr.scala	1	86.49%
journal/src/main/scala/com/evolutiongaming/kafka/journal/eventual/ReplicatedJournal.scala	1	82.14%
journal/src/main/scala/com/evolutiongaming/kafka/journal/FromConfigReaderResult.scala	1	50.0%
journal/src/main/scala/com/evolutiongaming/kafka/journal/util/GracefulFiber.scala	1	75.0%
replicator/src/main/scala/com/evolutiongaming/kafka/journal/replicator/CacheOf.scala	1	84.62%
replicator/src/main/scala/com/evolutiongaming/kafka/journal/replicator/ReplicateRecords.scala	2	75.44%

Totals
Change from base Build 6864775327:	0.03%
Covered Lines:	3279
Relevant Lines:	4009

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💛 - Coveralls