db: break heavy include chain from config.hh by extracting replication_strategy_type

Extract replication_strategy_type enum from locator/abstract_replication_strategy.hh
into a new lightweight header locator/replication_strategy_type.hh, and use it in
db/config.hh instead of the full abstract_replication_strategy.hh.

abstract_replication_strategy.hh pulls in a large transitive dependency tree
(schema.hh, mutation serializers, etc.) costing ~1.7s per file. With this change,
config.hh's incremental parse cost drops from 1.7s to 0.6s. Since ~85 files
include config.hh without also including database.hh (which would bring in these
deps anyway), this saves ~93s total CPU.

Speedup: part of a series measured at -5.8% wall-clock improvement
(same-session A/B: 16m14s -> 15m17s at -j16, 16 cores).

db/config.hh

@@ -15,7 +15,7 @@
 #include <seastar/util/program-options.hh>
 #include <seastar/util/log.hh>
 
-#include "locator/abstract_replication_strategy.hh"
+#include "locator/replication_strategy_type.hh"
 #include "seastarx.hh"
 #include "utils/config_file.hh"
 #include "utils/enum_option.hh"

locator/abstract_replication_strategy.hh

@@ -19,6 +19,7 @@
 #include "utils/sequenced_set.hh"
 #include "utils/simple_hashers.hh"
 #include "tablets.hh"
+#include "locator/replication_strategy_type.hh"
 #include "data_dictionary/consistency_config_options.hh"
 
 // forward declaration since replica/database.hh includes this file
@@ -38,13 +39,6 @@ extern logging::logger rslogger;
 using inet_address = gms::inet_address;
 using token = dht::token;
 
-enum class replication_strategy_type {
-    simple,
-    local,
-    network_topology,
-    everywhere_topology,
-};
-
 using replication_strategy_config_option = std::variant<sstring, rack_list>;
 using replication_strategy_config_options = std::map<sstring, replication_strategy_config_option>;
 

locator/replication_strategy_type.hh

@@ -0,0 +1,20 @@
+/*
+ * Copyright (C) 2015-present ScyllaDB
+ */
+
+/*
+ * SPDX-License-Identifier: LicenseRef-ScyllaDB-Source-Available-1.1
+ */
+
+#pragma once
+
+namespace locator {
+
+enum class replication_strategy_type {
+    simple,
+    local,
+    network_topology,
+    everywhere_topology,
+};
+
+} // namespace locator