Since the MigrationManager is using a constant value as a lock session id, which is what DistLockHandle is, there is no longer a need for MigrationManager::CollectionMigrationsState to store this value. DistLockHandle will always be equal to MigrationManager::_lockSessionID for every balancer instance.
Remove the MigrationManager::CollectionMigrationsState struct's distLockHandle variable, and clean up the MigrationManager code a bit because it no longer needs to handle it.
Get rid of the constant lockSessionID value per balancer instance. This commit is then revertible. This would probably involve creating the MigrationManager::CollectionMigrationState objects in MigrationManager::startRecoveryAndAcquireDistLocks
- has to be done before
SERVER-25758 Add unit testing for new variation of distlock unlock function