when updating a container, drop the single-container cache

This can cause incredibly weird and hard-to-debug problems if the previous
container value is still in the cache after an update, and in particular,
can result in having a stale container value cached after a roaring import
that modified the container. Coupled with another bug which could corrupt
containers on a delete, this produces a very strange bug where a value is
present in a fragment, but an attempt to delete it reports failure.

roaring/containers_btree.go

@@ -189,6 +189,8 @@ func (btc *bTreeContainers) Repair() {
 // replace the given container.
 func (btc *bTreeContainers) Update(key uint64, fn func(*Container, bool) (*Container, bool)) {
 	btc.tree.Put(key, fn)
+	btc.lastKey = ^uint64(0)
+	btc.lastContainer = nil
 }
 
 // UpdateEvery calls fn (existing-container, existed), and expects

roaring/containers_slice.go

@@ -210,6 +210,7 @@ func (sc *sliceContainers) Update(key uint64, fn func(*Container, bool) (*Contai
 			sc.insertAt(key, nc, i)
 		}
 	}
+	sc.invalidateCache()
 }
 
 // UpdateEvery calls fn (existing-container, existed), and expects