Semaphore table is only used by a core service but it depends on system install

In system_schema() we create a semaphore table. We should change Drupal\Core\Lock\DatabaseLockBackend to only create a table if necessary - this means that alternate lock implementations can safely remove the semaphore table and know that if the site swaps back to the DatabaseLockBackend the table will be created.

Also making this change encapsulates the storage to the backend which is good for general sanity.