Job coordinator of the table may have three states:

1) jobCoordinator == 0: the table was initialized, but no migration
   has ever been started for this table
2) jobCoordinator > 1: a table migration is in progress right now. The
   caller should join *jobCoordinator to help finishing the migration.
3) 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