This patch makes operation cache invalidate advisory rather than
immediate. Operations set a mark on cache entries meaning "this entry is
no longer valid", then the entry is removed next time the operation
is looked up.
This breaks the loop (now the cache can remove operations, but operations
can't remove cache entries), so it should be safer (I think). Everything
is inside a mutex, at least.
see https://github.com/libvips/libvips/issues/1484
d88ce970b7