Add state data cache and successor table. Remove keys table

* Adds a cache for the most recently validated ledger state
* Replaces the keys table with successor table
* Adds support for ledger diffs in the database
* Removes BackendIndexer

src/backend/SimpleCache.h

@@ -0,0 +1,60 @@
+#ifndef CLIO_SIMPLECACHE_H_INCLUDED
+#define CLIO_SIMPLECACHE_H_INCLUDED
+
+#include <ripple/basics/base_uint.h>
+#include <ripple/basics/hardened_hash.h>
+#include <backend/Types.h>
+#include <map>
+#include <mutex>
+#include <shared_mutex>
+#include <utility>
+#include <vector>
+namespace Backend {
+class SimpleCache
+{
+    struct CacheEntry
+    {
+        uint32_t seq = 0;
+        Blob blob;
+    };
+    std::map<ripple::uint256, CacheEntry> map_;
+    mutable std::shared_mutex mtx_;
+    uint32_t latestSeq_ = 0;
+    std::atomic_bool full_ = false;
+    // temporary set to prevent background thread from writing already deleted
+    // data. not used when cache is full
+    std::unordered_set<ripple::uint256, ripple::hardened_hash<>> deletes_;
+
+public:
+    // Update the cache with new ledger objects
+    // set isBackground to true when writing old data from a background thread
+    void
+    update(
+        std::vector<LedgerObject> const& blobs,
+        uint32_t seq,
+        bool isBackground = false);
+
+    std::optional<Blob>
+    get(ripple::uint256 const& key, uint32_t seq) const;
+
+    // always returns empty optional if isFull() is false
+    std::optional<LedgerObject>
+    getSuccessor(ripple::uint256 const& key, uint32_t seq) const;
+
+    // always returns empty optional if isFull() is false
+    std::optional<LedgerObject>
+    getPredecessor(ripple::uint256 const& key, uint32_t seq) const;
+
+    void
+    setFull();
+
+    // whether the cache has all data for the most recent ledger
+    bool
+    isFull();
+
+    size_t
+    size();
+};
+
+}  // namespace Backend
+#endif