We load+refresh the ChunkManager when the Balancer finds chunks to balance. And then we do it again in the MigrationManager to get collection version information to send with the moveChunk command

1)

• Add a chunkVersion and collectionVersion field in MigrateInfo.
• The balancer chunk selection policy represents chunks it selects to be balanced as MigrateInfo objects, which are sent to the Balancer to schedule.
• Then update MigrationType to serialize/parse for chunkVersion and collectionVersion, so config.migrations documents include them and MigrationManager recovery can build correct MigrateInfo objects to reschedule. (go look at type_migration.h/cpp on github for this – the class was checked in with version serialization/parsing, but it had to be removed later, so the code's already there in the file histories. Same for scoped_migration_request.h/cpp).
• Then, instead of loading up the ChunkManager again, just get the collection version from the MigrateInfo that's used to schedule a specific migration.

2)

• Pull out the ChunkManager code from the MigrationManager, as it is no longer needed if the MigrateInfo has both the chunk and collection versions.
• This looks like it will require some changes to manual moveChunk commands coming through from the mongos. Either the Balancer::rebalanceSingleChunk and Balancer::moveSingleChunk must load the ChunkManager now, or (I think better) the mongos should send chunkVersion and collectionVersion in the _configsvrMoveChunk command through to these balancer functions.