Fix ConcurrentMap::migrationInProcess() to handle initialization value (0) properly
Job coordinator of the table may have three states:
- jobCoordinator == 0: the table was initialized, but no migration has ever been started for this table
- jobCoordinator > 1: a table migration is in progress right now. The caller should join *jobCoordinator to help finishing the migration.
- jobCoordinator == 1: the table migration has been finished. It means that the table doesn't have any values in it anymore.
The patch actually fixes the map to consider case 1 as a situation, when
there is no migration.
CCBUG:411081