Add a realisations disk cache

Similar to the nar-info disk cache (and using the same db).
This makes rebuilds muuch faster.

- This works regardless of the ca-derivations experimental feature.
  I could modify the logic to not touch the db if the flag isn’t there,
  but given that this is a trash-able local cache, it doesn’t seem to be
  really worth it.
- We could unify the `NARs` and `Realisation` tables to only have one
  generic kv table. This is left as an exercise to the reader.
- I didn’t update the cache db version number as the new schema just
  adds a new table to the previous one, so the db will be transparently
  migrated and is backwards-compatible.

Fix #4746

src/libstore/binary-cache-store.cc

@@ -450,18 +450,43 @@ StorePath BinaryCacheStore::addTextToStore(const string & name, const string & s
 
 std::optional<const Realisation> BinaryCacheStore::queryRealisation(const DrvOutput & id)
 {
+    if (diskCache) {
+        auto [cacheOutcome, maybeCachedRealisation] =
+            diskCache->lookupRealisation(getUri(), id);
+        switch (cacheOutcome) {
+            case (NarInfoDiskCache::oValid):
+                debug("Returning a cached realisation for %s", id.to_string());
+                return *maybeCachedRealisation;
+            case (NarInfoDiskCache::oInvalid):
+                debug("Returning a cached missing realisation for %s", id.to_string());
+                return {};
+            case (NarInfoDiskCache::oUnknown):
+                break;
+        }
+    }
+
     auto outputInfoFilePath = realisationsPrefix + "/" + id.to_string() + ".doi";
     auto rawOutputInfo = getFile(outputInfoFilePath);
 
     if (rawOutputInfo) {
-        return {Realisation::fromJSON(
-            nlohmann::json::parse(*rawOutputInfo), outputInfoFilePath)};
+        auto realisation = Realisation::fromJSON(
+            nlohmann::json::parse(*rawOutputInfo), outputInfoFilePath);
+
+        if (diskCache)
+            diskCache->upsertRealisation(
+                getUri(), realisation);
+
+        return {realisation};
     } else {
+        if (diskCache)
+            diskCache->upsertAbsentRealisation(getUri(), id);
         return std::nullopt;
     }
 }
 
 void BinaryCacheStore::registerDrvOutput(const Realisation& info) {
+    if (diskCache)
+        diskCache->upsertRealisation(getUri(), info);
     auto filePath = realisationsPrefix + "/" + info.id.to_string() + ".doi";
     upsertFile(filePath, info.toJSON().dump(), "application/json");
 }