invalidate cache on multiple servers

The default cachestore for 2.4 and higher uses moodledata to store the cache - many sites use NFS for moodledata which is way too slow to use as a cache so the easy option would be to configure cachedir to use the servers local disk.

but... many of us also use multiple web servers and the cache doesn't support multiple locations - if the cache is cleared on one webserver it's not cleared on the other.