implemented prepared statement for postgres and sqlite

src/sql/statement_cache.cpp

@@ -0,0 +1,17 @@
+#include "matador/sql/statement_cache.hpp"
+
+namespace matador::sql {
+statement &statement_cache::acquire(const std::string &stmt, const connection &conn) {
+  std::lock_guard<std::mutex> guard(mutex_);
+  auto key = hash_(stmt);
+  auto it = statement_map_.find(key);
+  if (it == statement_map_.end()) {
+
+  }
+  return *it->second.statement_;
+}
+
+void statement_cache::release(const statement &stmt) {
+
+}
+}