Move handling of migration event source to solve bootstrap race.
Patch by Sergio Bossa and brandonwilliams, reviewed by Tyler Hobbs for
CASSANDRA-6648
Merge branch 'cassandra-1.2' into cassandra-2.0
Move handling of migration event source to solve bootstrap race.
Patch by Sergio Bossa and brandonwilliams, reviewed by Tyler Hobbs for
CASSANDRA-6648