[web/views/staticcontrollers] Make ConcatFilesHandler write to a tempfile
In order to be more resistant to errors such as ENOSPC, and against different
threads accessing the same static data, we:
- concatenate data files to a temporary file, and rename it once we're done
- delete the tempfile in case of errors
- protect the creation of the cache_concat file with a lock
Closes #3005547