[BUGFIX] Fix performance issue with mail spools

This patch removes the concatenation of the queued messages of the
spools in the __toString() methods which is needed for newer Symfony
releases.

Instead a simple identifier for representing the spool is returned.

Resolves: #92934
Releases: master, 10.4
Change-Id: I8b8559dcb8235ec14951c7233fa49f736bdc4860
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66902


Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Benni Mack <benni@typo3.org>