Remove MigrationManager::CollectionMigrationsState::distLockHandle variable or remove MigrationManager's constant value distlock lockSessionID

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.

OR: ------------

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