Add test to create multiple streams on seperate threads and in a transaction scope.
Merge branch 'master' into StressTest
Postgres - change Commits PK to CheckpointNumber and add unique index for (BucketId, StreamId, CommitSequence)
Oracle - change Commits PK to CheckpointNumber and add unique index for (BucketId, StreamId, CommitSequence)
Sqlite - add unique index for (BucketId, StreamId, CommitSequence)