chore: factor out rdb_load utilities into separate files (#4315)

* chore: factor out rdb_load utilities into separate files

rdb_load.cc is huge and contains many auxillary classes.
This PR moves DecompressImpl and ErrorRdb code into detail/

It also fixes minor bugs around error conditions with de-compression:
a. Do not check-fail on invalid opcode and return error_code instead.
b. Print correctly LZ4 errors.

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

* chore: fixes

---------

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

src/server/error.cc

@@ -0,0 +1,57 @@
+// Copyright 2024, DragonflyDB authors.  All rights reserved.
+// See LICENSE for licensing terms.
+//
+
+#include "server/error.h"
+
+#include <absl/strings/str_cat.h>
+
+using namespace std;
+
+namespace dfly {
+namespace rdb {
+
+class error_category : public std::error_category {
+ public:
+  const char* name() const noexcept final {
+    return "dragonfly.rdbload";
+  }
+
+  string message(int ev) const final;
+
+  error_condition default_error_condition(int ev) const noexcept final;
+
+  bool equivalent(int ev, const error_condition& condition) const noexcept final {
+    return condition.value() == ev && &condition.category() == this;
+  }
+
+  bool equivalent(const error_code& error, int ev) const noexcept final {
+    return error.value() == ev && &error.category() == this;
+  }
+};
+
+string error_category::message(int ev) const {
+  switch (ev) {
+    case errc::wrong_signature:
+      return "Wrong signature while trying to load from rdb file";
+    case errc::out_of_memory:
+      return "Out of memory, or used memory is too high";
+    default:
+      return absl::StrCat("Internal error when loading RDB file ", ev);
+      break;
+  }
+}
+
+error_condition error_category::default_error_condition(int ev) const noexcept {
+  return error_condition{ev, *this};
+}
+
+static error_category rdb_category;
+
+}  // namespace rdb
+
+error_code RdbError(rdb::errc ev) {
+  return error_code{static_cast<int>(ev), rdb::rdb_category};
+}
+
+}  // namespace dfly