NarInfoDiskCache: Keep BinaryCache.id stable and improve test

Fixes #3898

The entire `BinaryCaches` row used to get replaced after it became
stale according to the `timestamp` column. In a concurrent scenario,
this leads to foreign key conflicts as different instances of the
in-process `state.caches` cache now differ, with the consequence that
the older process still tries to use the `id` number of the old record.

Furthermore, this phenomenon appears to have caused the cache for
actual narinfos to be erased about every week, while the default
ttl for narinfos was supposed to be 30 days.

src/libstore/nar-info-disk-cache.hh

@@ -13,7 +13,7 @@ public:
 
     virtual ~NarInfoDiskCache() { }
 
-    virtual void createCache(const std::string & uri, const Path & storeDir,
+    virtual int createCache(const std::string & uri, const Path & storeDir,
         bool wantMassQuery, int priority) = 0;
 
     struct CacheInfo