If we are preloading files, also preload 'stat' result

[jjhursey]

• Enabled by default, can be disabled by setting the envar: SPINDLE_DISABLE_PRELOAD_STAT=1

[jjhursey]

When preloading files, we noticed that there is a stat call before the file is opened (to check permissions). This requires 2 calls to the root server to (1) cache the stat result, then later (2) to cache the file. By preloading the stat result with the file we combine those calls thus reducing the time to preload files.