Rewrite cache loader to async framework (#1193)

Fixes #1188

src/etl/CacheLoaderSettings.hpp

@@ -0,0 +1,60 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2024, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#pragma once
+
+#include "util/config/Config.hpp"
+
+#include <cstddef>
+
+namespace etl {
+
+/**
+ * @brief Settings for the cache loader
+ */
+struct CacheLoaderSettings {
+    enum class LoadStyle { ASYNC, SYNC, NONE };
+
+    size_t numCacheDiffs = 32;       /**< number of diffs to use to generate cursors */
+    size_t numCacheMarkers = 48;     /**< number of markers to use at one time to traverse the ledger */
+    size_t cachePageFetchSize = 512; /**< number of ledger objects to fetch concurrently per marker */
+    size_t numThreads = 2;           /**< number of threads to use for loading cache */
+
+    LoadStyle loadStyle = LoadStyle::ASYNC; /**< how to load the cache */
+
+    auto
+    operator<=>(CacheLoaderSettings const&) const = default;
+
+    /** @returns True if the load style is SYNC; false otherwise */
+    [[nodiscard]] bool
+    isSync() const;
+
+    /** @returns True if the load style is ASYNC; false otherwise */
+    [[nodiscard]] bool
+    isAsync() const;
+
+    /** @returns True if the cache is disabled; false otherwise */
+    [[nodiscard]] bool
+    isDisabled() const;
+};
+
+[[nodiscard]] CacheLoaderSettings
+make_CacheLoaderSettings(util::Config const& config);
+
+}  // namespace etl