Fix statcache race condition in cachestore_file

It's tricky to test this patch. You'd need xdebug enabled in your test set up.

1. Have a moodle  with a default caching settings 

2. Enable debugging by adding $CFG->debug = (E_ALL | E_STRICT);  and  $CFG->debugdisplay = 1; to your config.php

3. In your IDE stop execution of the script at line 427 of cache/stores/file/lib.php

4. Navigate to /course/management.php with edebug in debugging mode, in your IDE it will stop on the line 427, copy value of $file variable (don't continue)

5. In my case the value is /var/lib/sitedata/cache/cachestore_file/default_application/core_config/939-cache/939654e0580e0b66899e27f8c9b24bf5e490488f.cache

6. On my dev box edit a content of this file and remove everything, then save file.

7. Continue the script execution so it could fail at line  569 of cache/stores/file/lib.php

8. Confirm that no exception is thrown and you can load the page successfully, but see following debugging errors:
   Can not read data from cache file: /var/lib/sitedata/cache/cachestore_file/default_application/core_config/939-cache/939654e0580e0b66899e27f8c9b24bf5e490488f.cache
   Failed to unserialise data from cache file: /var/lib/sitedata/cache/cachestore_file/default_application/core_config/939-cache/939654e0580e0b66899e27f8c9b24bf5e490488f.cache. Data: