Fix EZP-23296: Template Block Cache has cache key collisions

The template cache blocks where using crc32 which has a high probability of collisions.
Feature has been changed to use md5 like viewcache already uses.

Note: To avoid that you have to many cache blocks take advantage of putting identical once in
own templates so they will share the case (file and location of block is part of the unique key).

_WARNING: If this is back ported, cache for template block will have to be cleared!_
1. I am not sure I grok this phrase
2. I am not sure the user understands that, if he is upgrading, all his cache-blocks will be expired, and he needs to prepare for the accompanying performance hit (plus it is a good idea to run cache-clear-and-purge scripts to remove chache files with old names