Cache key prefix for the bin-specific entry to track the last write.
const LAST_WRITE_TIMESTAMP_PREFIX = 'last_write_timestamp_';
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Cache!ChainedFastBackend.php/constant/ChainedFastBackend::LAST_WRITE_TIMESTAMP_PREFIX/8.1.x