lru_cache: restrict and annotate Send impl

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/tools/lru_cache.rs

@@ -100,7 +100,8 @@ pub struct LruCache<K, V> {
     _marker: PhantomData<Box<CacheNode<K, V>>>,
 }
 
-unsafe impl<K, V> Send for LruCache<K, V> {}
+// trivial: if our contents are Send, the whole cache is Send
+unsafe impl<K: Send, V: Send> Send for LruCache<K, V> {}
 
 impl<K: std::cmp::Eq + std::hash::Hash + Copy, V> LruCache<K, V> {
     /// Create LRU cache instance which holds up to `capacity` nodes at once.