Add forwarding cache (#1204)

Fixes #51.

src/etl/LoadBalancer.hpp

@@ -23,6 +23,7 @@
 #include "etl/ETLHelpers.hpp"
 #include "etl/ETLState.hpp"
 #include "etl/Source.hpp"
+#include "etl/impl/ForwardingCache.hpp"
 #include "feed/SubscriptionManager.hpp"
 #include "util/config/Config.hpp"
 #include "util/log/Logger.hpp"
@@ -72,10 +73,12 @@ private:
     static constexpr std::uint32_t DEFAULT_DOWNLOAD_RANGES = 16;
 
     util::Logger log_{"ETL"};
+    // Forwarding cache must be destroyed after sources because sources have a callnack to invalidate cache
+    std::optional<impl::ForwardingCache> forwardingCache_;
     std::vector<Source> sources_;
     std::optional<ETLState> etlState_;
     std::uint32_t downloadRanges_ =
-        DEFAULT_DOWNLOAD_RANGES; /*< The number of markers to use when downloading intial ledger */
+        DEFAULT_DOWNLOAD_RANGES; /*< The number of markers to use when downloading initial ledger */
     std::atomic_bool hasForwardingSource_{false};
 
 public:
@@ -164,7 +167,7 @@ public:
         boost::json::object const& request,
         std::optional<std::string> const& clientIp,
         boost::asio::yield_context yield
-    ) const;
+    );
 
     /**
      * @brief Return state of ETL nodes.