Changed clearing of cache to use hook_flush_caches.

These changes avoid the situation where multiple requests will try to
delete the saved assets at the same time and generate a bunch of PHP
warnings.
Added namespacing.
Added clear asset saver saved files item to admin menu "Flush all caches"
Renamed FLUSH_CACHE_LOCK to CLEAR_FILES_LOCK.

Name is more appropriate and matches the related function name.
Added locking to saving of files.

This should avoid the situation where near-simultaneous can interfere
with each other when trying to save assets.