[BUGFIX] Too many tags by identifier in CacheBackends

This applies to Apc/Memcached/Wincache/Xcache backends.

After an array_merge the values aren't unique. This leads to duplicate
tags per identifier. This patch changes that and also moves the
findTagsByIdentifier call out of the foreach loop.

Resolves: #59587
Releases: 6.3, 6.2, 6.1
Change-Id: Id31e16fa4bba11038ba692a483fb9a33808d95fa
Reviewed-on: https://review.typo3.org/31519
Reviewed-by: Markus Klein
Tested-by: Markus Klein