More flexible caching (#1101)

internal/caching/caches.go

@@ -0,0 +1,15 @@
+package caching
+
+// Caches contains a set of references to caches. They may be
+// different implementations as long as they satisfy the Cache
+// interface.
+type Caches struct {
+	RoomVersions Cache // implements RoomVersionCache
+	ServerKeys   Cache // implements ServerKeyCache
+}
+
+// Cache is the interface that an implementation must satisfy.
+type Cache interface {
+	Get(key string) (value interface{}, ok bool)
+	Set(key string, value interface{})
+}