[BUGFIX] Prevent filenames longer than 255 characters

The temporary file created in the (Simple)FileBackend should not
exceed 255 characters which happened with the addition of
microtime and process id. Therefor the cache identifier is now
hashed in the temporary file name.

Change-Id: I88b03998728f7b3a7ada999cf8650fefeea30182
Releases: 2.1, master
Fixes: #55306