[9.x] Always use the write PDO connection to read the just stored pending batch

[corneyl]

Setting the sticky option to false for a database connection with separate read/write database nodes, results in problems with finding a batchable job right after creation.

This can happen because the read query is executed on a replicated read-only node directly after writing to the write node. If the select query is executed faster than the database replication takes place, then no batch is found. In that case an exception is thrown because jobs are added to a null batch.

This PR fixes this problem by always using the write PDO connection for finding the just created batchable job in the store() method in DatabaseBatchRepository.php.

[taylorotwell]

Probably easier to just always use write PDO in this situation. We can't change a contract method signature on a patch release.