Improve message when lock is held by some other client

[sky-andremartins]

Currently the message logged when there is another client holding the lock does not contain that client id because it relies on the toString method from ResultSet and thats not convenient. https://github.com/sky-uk/cqlmigrate/blob/d8cad30b6ca75796921e9b1f5e5ca26969c34abf/src/main/java/uk/sky/cqlmigrate/CassandraLockingMechanism.java#L80