add function to invalidate one opcache file, use it if possible #9885

Issue #9885 appears to be triggered by ownCloud invalidating the entire
PHP opcache. Testing indicates it can be avoided by only invalidating the
single file that was written from the opcache, instead of clearing the
whole thing. In general it is more efficient to invalidate only the single
file that was changed, rather than the whole cache.

This adds a deleteFromOpcodeCache() function which invalidates a single
file from the opcache if possible, returning true on success. It also
changes writeData() in config.php to try using deleteFromOpcodeCache()
and only fall back on clearOpcodeCache() if that fails.