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.cpp

@@ -0,0 +1,96 @@
+#include <backend/SimpleCache.h>
+namespace Backend {
+
+void
+SimpleCache::update(
+    std::vector<LedgerObject> const& objs,
+    uint32_t seq,
+    bool isBackground)
+{
+    std::unique_lock lck{mtx_};
+    if (seq > latestSeq_)
+    {
+        assert(seq == latestSeq_ + 1 || latestSeq_ == 0);
+        latestSeq_ = seq;
+    }
+    for (auto const& obj : objs)
+    {
+        if (obj.blob.size())
+        {
+            if (isBackground && deletes_.count(obj.key))
+                continue;
+            auto& e = map_[obj.key];
+            if (seq > e.seq)
+            {
+                e = {seq, obj.blob};
+            }
+        }
+        else
+        {
+            map_.erase(obj.key);
+            if (!full_ && !isBackground)
+                deletes_.insert(obj.key);
+        }
+    }
+}
+std::optional<LedgerObject>
+SimpleCache::getSuccessor(ripple::uint256 const& key, uint32_t seq) const
+{
+    if (!full_)
+        return {};
+    std::shared_lock{mtx_};
+    if (seq != latestSeq_)
+        return {};
+    auto e = map_.upper_bound(key);
+    if (e == map_.end())
+        return {};
+    return {{e->first, e->second.blob}};
+}
+std::optional<LedgerObject>
+SimpleCache::getPredecessor(ripple::uint256 const& key, uint32_t seq) const
+{
+    if (!full_)
+        return {};
+    std::shared_lock lck{mtx_};
+    if (seq != latestSeq_)
+        return {};
+    auto e = map_.lower_bound(key);
+    if (e == map_.begin())
+        return {};
+    --e;
+    return {{e->first, e->second.blob}};
+}
+std::optional<Blob>
+SimpleCache::get(ripple::uint256 const& key, uint32_t seq) const
+{
+    if (seq > latestSeq_)
+        return {};
+    std::shared_lock lck{mtx_};
+    auto e = map_.find(key);
+    if (e == map_.end())
+        return {};
+    if (seq < e->second.seq)
+        return {};
+    return {e->second.blob};
+}
+
+void
+SimpleCache::setFull()
+{
+    full_ = true;
+    std::unique_lock lck{mtx_};
+    deletes_.clear();
+}
+
+bool
+SimpleCache::isFull()
+{
+    return full_;
+}
+size_t
+SimpleCache::size()
+{
+    std::shared_lock lck{mtx_};
+    return map_.size();
+}
+}  // namespace Backend