Use a more space/time-efficient representation for the eval cache

src/libstore/local-store.cc

@@ -588,7 +588,7 @@ uint64_t LocalStore::addValidPath(State & state,
         (concatStringsSep(" ", info.sigs), !info.sigs.empty())
         (info.ca, !info.ca.empty())
         .exec();
-    uint64_t id = sqlite3_last_insert_rowid(state.db);
+    uint64_t id = state.db.getLastInsertedRowId();
 
     /* If this is a derivation, then store the derivation outputs in
        the database.  This is useful for the garbage collector: it can

src/libstore/sqlite.cc

@@ -61,6 +61,11 @@ void SQLite::exec(const std::string & stmt)
     });
 }
 
+uint64_t SQLite::getLastInsertedRowId()
+{
+    return sqlite3_last_insert_rowid(db);
+}
+
 void SQLiteStmt::create(sqlite3 * db, const string & sql)
 {
     checkInterrupt();

src/libstore/sqlite.hh

@@ -26,6 +26,8 @@ struct SQLite
     void isCache();
 
     void exec(const std::string & stmt);
+
+    uint64_t getLastInsertedRowId();
 };
 
 /* RAII wrapper to create and destroy SQLite prepared statements. */