implemented prepared statement for postgres and sqlite

include/matador/sql/statement_cache.hpp

@@ -0,0 +1,36 @@
+#ifndef QUERY_STATEMENT_CACHE_HPP
+#define QUERY_STATEMENT_CACHE_HPP
+
+#include "matador/sql/statement.hpp"
+
+#include <memory>
+#include <mutex>
+#include <string>
+#include <unordered_map>
+
+namespace matador::sql {
+
+class connection;
+
+struct cache_info
+{
+  std::unique_ptr<statement> statement_;
+  size_t connection_id_;
+};
+
+class statement_cache
+{
+public:
+  statement& acquire(const std::string &stmt, const connection &conn);
+  void release(const statement &stmt);
+
+private:
+  mutable std::mutex mutex_;
+  size_t max_cache_size_{256};
+  std::hash<std::string> hash_;
+  using statement_map = std::unordered_map<size_t, cache_info>;
+  statement_map statement_map_;
+};
+
+}
+#endif //QUERY_STATEMENT_CACHE_HPP